Click on the links to the left for each con...
So when folks find out that I go to scifi conventions, I get the same two questions:
1. What do they do there, and
2. Do you dress up?
Ok the answer to #2, is no. But I haven't ruled it out.
To tackle question #1; now this is from my perspective...I go mostly for media guests.
There are two types of cons; I'm sure they have real names but for now I will call them the Expo-Type Cons and The Structured Cons. Cons are held at a hotels and convention centers.
At both types of cons there are what's called panels, Q&As, or presentations. This is where the guest is up on a stage, talks about their experiences with acting (or anything else they fancy), and fans can ask questions. Some panels are just fans getting together to discuss a certain topic, such as a show or current event. There are autograph and photo opportunities. There is also a vendor's area where you can find some interesting stuff to buy. There is always a costume contest.
Let's start with the Expo-Type cons. There is one ticket price for entry. These are cons where there is a signing area where all the guests sit at tables to sign autographs. For a fee (sometimes outrageous) you either bring an item to have signed, or choose an 8x10 photo that the actor has available to sign. The cost can be anywhere from $20 & up (way up for some). Depending on the actor, you can also take a photo with them with your own camera for free, for free with autograph, or for an extra fee. Some will also have a professional "photo op" schedule where a photographer takes the photo. The thing I like about these cons is that you can just walk up to the table and say hi & chat with the actor if that's all you want. Sometimes these cons have featured guests that are only available during autograph & photo op sessions. Usually a bazillion people go to them (it's cheaper to get in) and it's completely insane and a hell of a lot of fun. Most of the folks are wearing some kind of costume. I've seen everything from Klingons to Jesus Christ. The panels are general admission. Examples of this type of con are Megacon, FX Show, and Dragoncon (the East Coast biggie). These cons are typically a combination of comic book, anime, gaming, music, & scifi.
Then there's The Structured Con. These are not quite so chaotic, and have various ticket prices. Most of the day you attend panels by the different guests. Seating is dependent on ticket price, there is both reserved seating and general admission seating (towards the back, of course). Autograph & photo ops are scheduled at certain times. The Structured Con also has, for an additional fee, banquets, breakfasts, and cocktail parties where you get to meet & greet the guests. These events are often exclusive to, and/or included with, The Most Expensive Ticket. Although more costly and less access to the guests, it's a refreshing chance of pace from The Expo-Type Con. I enjoy both equally. Examples of this type of con are Eyecon, and the Creation cons. Some cons, like Vulcon, are a combination of both types of cons; there are featured guests with only autograph/photo op sessions and there are bonus guests in a signing area. There is usually gaming & comic book guests at these cons.
I'm sure there's much more info to cover, but my brain is telling me to transition and do something else right now. Hope this helps.