If you want to waste time and frustrate yourself, start a conversation/debate about soccer-football positions, what you call them, what that means, and what that implies about their roles.
This is probably why there is so little intelligent discussion about these things in the English-speaking world, if for no other reason than the fact that the very people who are purported to have invented the game long ago decided that tactics talk was somehow inherently cynical and have always seemingly been a half-step behind the rest of the world (and explains the rather off-putting nationalism that takes over the English sporting press any time a major international tournament rolls around).
Similarly, be wary of anyone who speaks of "Total Football" as a stand-alone concept. Here's the truth about "Total Football": it is a 40-year-old concept that has been fully integrated into the global culture and tactical awareness of the game. Anyone gushing about "total football" in 2011 is likely a neophyte to thinking of the game beyond simplistic levels, or are stuck in the rigid ways in which we approach "American" sports. That may seem harsh but I myself was guilty of this until just a few short years ago myself.
What do these things have to do with anything? Well, I bring it up because of the ongoing difficulty I have with talking about defensive positions, and comparing "3-man" defenses to "4-man" defenses, and what is a Fullback and what is a Wingback.
So let's begin with a glossary:
Fullback: to understand what a Fullback is you have to understand the 2-3-5 formation, from which modern soccer evolved. No one has played the 2-3-5 for quite some time, but even so the terminology of that formation has had the odd habit of sticking around. Obviously, in the 2-3-5 you had 5 forwards, 3 half-backs, and 2 fullbacks. Over time, the center halfback, shortened to center half, dropped back to become the central defender, the fullbacks taking up wider positions. Eventually, the center half was given a partner, and fullbacks took up even wider positions; this also explains the confusing British habit of referring to central defenders as "center halves". At any rate, Fullbacks are the outside defenders in a "back 4" defense.
Center back: See above. The old "center halves" ought to really be called center backs. And here's a twist: in a "three-man" defense, all three defenders are usually considered "center backs". On the surface, this doesn't make much sense, but it eventually will, and I hope to help explain that.
Wingback: This term bothers some people, but the "wingback" is evolved from the fullback, and is best described as the wide defensive mids who support a more central back three in a "3-man" defense. This leads some to question if the back 3 is really a more defensive formation, with wingbacks essentially being fullbacks lined up in the midfield. More on that in a bit.
CDM: Central defensive midfielder. Yes, he (or she, i guess I should be equitable) is probably best thought of as a defensive player. This may bring on the question of what happens if a team uses two CDM's, as in a 4-2-3-1? While in some cases this does throw a wrench into my way of thinking, it is also worth considering that in many 2-man CDM partnerships, one of the two is really more of a deep-lying playmaker, while the other is more of the bulldog type; and it would be that bulldog type we would include here.
Back 4 vs. back 3
If you want to upset people, suggest that a back 3 is really a MORE defensive tactic. I don't want to turn this into comprehensive breakdown of ramifications of "spare defenders" and single-striker systems and midfield match ups and width, but given that in all but the rarest exceptions the outside mids are expected to help provide wide defensive cover (as explained above in the "wingback" definition) this seems to have some credence. The key to running a back three is often on how free your wingbacks are to attack, and your opponent running a narrow system (like a midfield diamond or 4-3-1-2) can help this cause. We saw this is Serie A last season, with narrow formations almost de rigeur but a few clever teams decided they could run 3-4-1-2 (or similar shapes) and allow their wingbacks to run rampant.
Before the season, I suggested that the best way to look at it is that a "back 3" really includes 3 players on the deepest defensive layer and a "back 4" uses 2 players on the deepest defensive layer. Both use essentially 5 defensive players, just arranged in different configurations.
What we are seeing more and more is that the CDM is becoming more of a "sweeper," which is to say an "auxiliary" center back free to push forward into the midfield. This has led to back 4 systems becoming more and more hybridized between a "back 3" with wingbacks and "back 2" with fullbacks and a CDM. The CDM evolved to counter the classic "#10", "enganche" or "playmaker" but that player has gradually faded away with the decentralization of the playmaker role. But the CDM has stuck around and seems to be evolving into a reprise of the "libero"
Wingbacks vs. Fullbacks:
And here's the crux of the conversation. Asking whether the difference is if wingbacks are really midfielders and fullbacks defenders entirely misses the point, as they are essentially the same player, the difference being the freedom they have to attack based upon who is in the middle of defense and who they have in front of them on the other team. An opponent with a tridente of a center forward and two forward wingers can pin those wingbacks back, leaving 3 CB's to cover one man in the middle and wingbacks little freedom to go forward; and you find yourself bunkering with 5 defenders. Of course you can respond by freeing a CB into a "libero" and have him essentially become a midfielder, and now you have a back 4 with a spare man - which is always the goal on defense. A opponent running a diamond would allow those wingabcks a lot of freedom, with their defensive responsibilities primarily being against the opposing fullbacks (although the diamond generally calls for fullbacks to push high in support of the narrow midfield); you have 3 defenders against 2 strikers in the middle, leaving you the desired spare man.
But I digress, as in the interest of keeping this to some reasonable length it simply does not make sense to break down every possible tactical matchup, what I provide above is only for the means of illustrating the greater point.
On the importance of Fullbacks:
Fullbacks are pretty important, and the level and style of your fullbacks seems to serve as a pretty good litmus test of the overall quality of the soccer involved. I think that it could be fairly said that we don't see the type of fullback play at the MLS level that we see in other parts of the world, and this speaks to the overall level of MLS. Fullbacks are so often some of the quickest, fittest, and more well-rounded players in soccer, and are often really box-to-box players.
But this isn't to say that MLS fullbacks are unimportant, as few things could be further from the truth. One thing I have observed from perusing the chalkboards of Sounders matches is how many touches the fullbacks in those matches get, and how it is usually the case that the fullbacks are 2 of the top three in touches in a match - with Alonso at CDM being the other. What this seems to illustrate is the impact of that second layer of defense, the three higher players in the "W", and how much of an impact they have on the game in both possession and defending.
If there is anything "total football" has taught us is not that anyone plays any position, but that everyone plays TWO positions. Fullbacks retain their original name, but they are in fact also wide midfielders. The CDM retains his original name, but he is in fact also an ACB (auxiliary center back). When a fullback pushes forward, it is not the outside mid who covers for him - as outside mids are also forward wingers - but the center back who slides outward, the center mid(s) thus sliding back to cover.
We have become accustomed to multiple roles within one position and multiple layers within the forward bands and the midfield bands, but this way of thinking has seemingly not yet crept in to how we view the back third of the pitch. Perhaps it is high time that changes.