The general job of a restaurant host is to meet, greet, and seat customers. Therefore it is an excellent entry-level job for someone without a lot of restaurant experience (or any). The host should be friendly and courteous, as well as organized and comfortable multi-tasking, knowing how to handle busy shifts.
The bartender is the top position of the front of house staff. Depending on the type of restaurant, the bartender may be responsible for getting the rest of the staff drinks for their tables (service bar) as well as the serving of customers who choose to eat at the bar.
A restaurant server is the customer service rep for your restaurant. A good restaurant server can convert any customer into a regular, while a poor server can just as easily turn customers away. A restaurant server job description can include many different duties beyond just serving food.