First off, please understand that I am simplifying things radically here. We are talking about stereotypical conventions, after all, and I wouldn't want to imply that one Blake's 7 con is the same as a Star Trek con, or indeed the same as another Blake's 7 con. I'm also going to ignore side issues like filkcons, relaxacons, professionally-run cons and so on. That's not to say that they aren't important, it's just that they aren't really relevant to this discussion, and I can live without the complication for what is essentially a Beginner's Guide to Conventions.
Now, what I'm calling a media con (and to be fair, that's what they mostly call themselves) is a convention devoted to Science Fiction and Fantasy as depicted in televisual media (i.e. film and TV, with a smattering of radio thrown in from time to time by those who know better). Generally (and remember we are talking stereotypes here) they have actors as guests, though sometimes with other interesting people associated with TV SF; a certain executive producer springs to mind as a popular choice at Babylon 5 conventions, for example. The guests usually get to stand up on stage and do their thing, whether it be interview, question and answer session, talk or whatever, and tend to have most of the membership attending their programme items. Con-goers get to listen to (and sometimes talk with) the guests, watch videos, wait in huge queues for autographs and generally have a good time.
The other type of convention I'm calling mainstream mostly for lack of anything else to call it. That plus the minor detail that book-based fandom was holding conventions some while before television had got into most households! Anyway, a mainstream con as you will have gathered is a convention devoted to SF and Fantasy as it appears in books and magazines. Generally they have authors as guests, with a small sprinkling of other important people such as artists. Again, the guests will get to stand up on stage and do their thing, but though a large fraction of the membership will turn up, by no means everyone always goes to guest items. There is quite a lot of non-guest-related programming going on at a mainstream con, in various forms, quite apart from which mainstream fans seem to spend more time in the bar than their media equivalents!
A major difference between the two styles of convention seems to be the way that the programme is directed. At a media con, there are often a sizeable number of guests present, and the programme will be almost entirely devoted to them. At a mainstream con, there will usually only be a couple of guests of honour (even worldcons rarely go beyond five guests, including the Fan Guest and the Toastmaster), and while the programme may focus on them it will also contain a lot of items that involve no guests at all. This appears to be a major cultural thing; there is a large "Them and Us" divide at media cons between the members and the guests, which is much less apparent at a mainstream con. Conversely mainstream con-goers are less likely to be satisfied with being passive consumers, and tend to become part of the programme they are attending.
Quite apart from this, a medium to large mainstream con will generally have quite a lot of authors who have just turned up on spec, as it were, because they have a new book to push or just because they like going to conventions. This means that there are authors everywhere, on the programme and in the bar, which makes striking up random conversations an interesting proposition and tends to increase the sense of community; these people are always there to talk to. If you're really unlucky, they'll swipe bits of you for characters in their next book!
Seriously, though, a fair number of fans are on chatting terms with a fair number of authors. Most mainstream fans can happily regale you with tales of odd conversations with someone famous (like explaining to Diana Wynne Jones that she hadn't offended me, I was just too shy to talk to her, but enough name-dropping). It helps that a lot of authors were fans too in their time, and still have their stories of talking with the old masters. There is a real sense of continuity that can sometimes be missing from media cons.
Media cons don't get the same sort of effect of extra not-really-a-guest actors turning up on spec, mostly because actors are obliged to charge appearance fees (quite apart from most of them living in the US, which makes popping over to a UK con a bit of an expensive proposition all round), but also because some actors really aren't very interested in the genre. This isn't a bad thing -- I for one am glad that people have different tastes, even if I can't understand why anyone enjoys horror movies -- but is an inevitable side-effect of the fact that actors are there to do a job -- act -- rather than to write for the love of it. That some of them do love it is the miracle, and a very gladdening one too.
Anyway, I hope that's given a flavour of the broad differences between these two branches of fandom. Please do remember that we are speaking gross generalities here, and that you will find a few mainstream cons acting in a media style, and a few media cons acting in a mainstream style, as well as events like The Wrap Party (an end-of-Babylon 5 celebration that took place back in August 1998) that deliberately set out to blur the distinctions. If you belong to one branch of fandom, it's worth dipping into the other from time to time, just to remind yourself that there is more than one way to do things.
Mind you, I suppose I would say that as a filking conrunner who has published his own zine and hacks Perl for a living!