At its heart a Rogue is a lot more than what a lot of things try to narrow it down to. A Rogue is an "Every Man", a Jack of All Trades but Master of None. They're a person who don't go down the beaten path, they stray off. They may or may not follow the rules, but whether they do or don't it is not just because they are the rules. The Rogue has its own reasons for doing whatever it does. The Rogue can fight, but he is not a fighter. The Rogue can often pick locks and steal things, but they are not a thief. A Rogue can sweet talk his or her way in and out of trouble as quick as almost anyone, but they are not a bard. A Rogue can have a heart of gold, or they can be as mercenary as they come.
In a lot of ways a Rogue is an easy term to apply to characters you see in movies and books. I mean, so many things can be covered by the term, that it can be kind of ridiculous. The only problem is though is that while I said a Rogue is not any one of those things above, the truth is the Rogue is all of them. In 'story' terms a Rogue is usually one of the better fighters, though perhaps not the best (especially when it comes to straight up fights). They will often double as a thief, and why not especially when many thieves are rogues, but if a Thief is also present they will be the back up to said thief.
It is that that you have to keep in mind though when making a Rogue. Whether for a story or a RPG, Rogues are actually kind of hard to make and play with. Points (for games) come at a premium, and you have a LOT of things you need to be able to cover. Because your character needs to be able to make it on their own, to at least some degree. A rogue just isn't a rogue after all if they are dependent on others for survival.
Details wise, Rogues tend to be a bit older, I'm not talking middle aged or anything necessarily, but they have to be old enough to have experience. The worldliness and savvy that comes with it are one of the defining elements of a rogue. Without it, you have a fighter/thief, a potential future rogue yes, but not quite one now. For example (yep, Star Wars again) look at Star Wars. Luke is about 19, meaning Leia is as well. Obi-Wan is clearly doing the "old mentor" route, so he is older. The other PC on the scene though is Han, playing the Rogue. His character comes in in the mid to late 20s. His career started young, but even with that he is the oldest of the group (aside from Ben, and possibly Chewie but Wookies age weird) to help further explain the experience he has. It grounds him in reality, and helps give him that charm he has.
When it comes to tricky situations, Rogues generally aren't the "straight up the middle" types. (Regardless of what Han may or may not do in A New Hope ;) ). They are, after all, not straight up fighters. In stead they tend to do the more out of the box type thinking. They solve combat the way they do every problem, with a combination of brains, finesse, and the proper application of force at the right moment. This, story wise, is often part of what makes them so bad ass because at some point we see them take on a situation with a plan, and in that moment when the plan is pulled off they seem stronger than they may actually be.
Essentially, I guess what I am trying to say is that Rogues are hard to categorize. It is more a way of life, a badge earned after so many years of experience and living that life than anything else. It is not a specialist, but a highly competent generalist. It is not a lot of things, but it is a little bit of everything. They are hard to pull off in games, but when you do you'll have done something special. They're hard to pull off in stories, but when you do you'll have a memorable character that people will probably like a whole lot. There are specialized kinds of rogues, just like with the Fighter. The Elite for instance can easily be a Rogue just as easily as anything else. It just is a bit weirder with that.
I'll go into the Rogue variants and specialists later, even if it is just other types that often can also take the Rogue Mantle as well.