Titanic is the story of the Titanic, obviously, but also about a mismatched rags/riches love affair with the Titanic as the backdrop. Truth be told, you could chop out the ridiculous love story and end up with a very watchable 2 hour recreation of the sinking. In fact, I would have done so had I been in charge, and instead of a pile of sentimental goo focus on the myriad of human stories associated with the ill-fated voyage. Let's be honest; the ship is the real star of this show, and all the actors are just trying to keep up.
I can't honestly remember any of the character names, so I'll just stick with the actor names. Leonardo Di Caprio is himself; if you like him you'll like his character. I don't like Leonardo at all, but reportedly some do. Kate Winslet is much better, but largely wasted in the role of rebellious princess. The rest of the characters are Leo's friend and shipmate, who disappears quickly, and a bunch of evil aristocrats. Man, are they evil, but so well dressed and well mannered. Evil aristocrats are a dime a dozen in modern movies, but I can never tell whether I'm supposed to be for or against them. Billy Zane's character is hilarious. I have no idea if he's even supposed to be a real person, or just some sort of laughably evil cyborg. His script must have said 'do the most evil thing possible' over and over. My favorite part is when Winslet runs away and he says something to the effect of "I've lost something that belongs to me". And you think "OH, he means Kate Winslet, haha." Lest we forget, Kathy Bates plays another aristocrat, but a good one. Bates is usually the best character in every movie she's in, and this one is no exception. Makes you wish it was about her instead of Leo.
The movie takes a hiatus from all this for about an hour to show the sinking. This part is quite impressive, with special effects that hold up well even now. Unfortunately, by the 20th time you see loads of terrified people swarming towards the dry end and falling to their icy deaths, it starts to get a little awkward. I mean, are we supposed to be enjoying watching these people's pathetic last moments? It's a problem of cinema I suppose, with its photorealism.
In sum, the best part of Titanic is the Titanic, and the worst part is the story and most of the characters.