A sociopath is typically defined as someone who lies incessantly to get their way and does so with little concern for others. A sociopath is often goal-oriented (i.e., lying is focused - it is done to get one's way). Sociopaths have little regard or respect for the rights and feelings of others. Sociopaths are often charming and charismatic, but they use their talented social skills in manipulative and self-centered ways (see lovefraud, for more on sociopaths).
A compulsive liar is defined as someone who lies out of habit. Lying is their normal and reflexive way of responding to questions. Compulsive liars bend the truth about everything, large and small. For a compulsive liar, telling the truth is very awkward and uncomfortable while lying feels right. Compulsive lying is usually thought to develop in early childhood, due to being placed in an environment where lying was necessary. For the most part, compulsive liars are not overly manipulative and cunning (unlike sociopaths), rather they simply lie out of habit - an automatic response which is hard to break and one that takes its toll on a relationship (see, how to cope with a compulsive liar).
The terms Pathological Liar, Habitual Liar and Chronic Liar are often used to refer to a Compulsive Liar
Take a quick survey and see how your lying compares with others -compulsive lying quiz