There's actual some differences between the living room and the sitting room. Many people don't know this and tend to use the two interchangeably. It's not absolutely wrong though as they are very similar. I think it's more of the purpose they serve that makes the difference.
The living room is usually placed close to the place for dinning, the dining room which usually extends from it. It's where the family can gather around, maybe during the evenings to watch their favorite shows or operas. It's where they get to relax and have fun together. The living room is usually the first place you see when you open the front door.
The living room would usually contain a set of furniture, the sofas, the center table, flower vase and almost certainly a television. The living room is usually the biggest part of the house as it's where you welcome your guests and visitors.
The living room is mostly given a sense of grandeur and a lot of things are considered like the lightning, the painting has to be rich depending on the style of the owner. It's common to see paintings on the walls.
The sitting room differs from the living room in that it can be placed anywhere in the house and not necessarily close to the dining room. It's simply a place where one can burn some steam. It's usually the place where you can see a bookshelf on the wall and a fireplace.
In the sitting room, there's no need for an elaborate furniture set. Just one sofa can do. It's a place where someone can come to read a book or surf the net. To some, it can serve as a place where they can go to have some quiet time. Time when they can think about things that have been disturbing them or lingering on their minds. .
The sitting room doesn't necessarily need a television. Many people tend to avoid having one there. It may be the place where you go to wait as the meal is being prepared. Some houses I've seen tend to have it close to the kitchen.
The sitting room is not usually the biggest part of the house which contracts with the living room. It might just have enough room for a sofa where everyone can relax - by everyone I mean every family member as the sitting room is mostly used by family - and have time together, a bookshelf for storing books, a fireplace and/or a computer desk. I've seen people who like to use the sitting room like an office.
Sometimes however, one room can serve as both the living room and the sitting room and would usually have a blend of both features.