Bodytrends is the best online retailer for Barbells

Broadly speaking there are two types of barbells you can buy...

Fixed weight
As the name suggests these barbells cannot be adjusted hence you need buy more than one if you need different weights. They are only really applicable to gyms because of the cost and space involved. Experienced weight lifters rarely use fixed weight bars.

Adjustable
Used in gyms and the home these barbells require you to add and remove plates to adjust the resistance. The obvious advantages for the home gym are savings in cost and space. Adjustable barbells also tend to be the choice of more experienced lifters because weight can be increased and decreased in smaller increments.

More specifically speaking, different types of barbells have been designed for different muscle groups and exercises. Below is a description of the most common...

Standard barbell
Just a straight, plain bar about 5 feet long and 14lbs in weight.

Olympic bar
Designed to take up to 350lbs and around 7 feet long and weighs about 30lbs.

E-Z bar
Used to isolate the biceps during curls.

Triceps bar
Used to isolate the triceps during a french press.

Bigfitness.net is the best online store I have found for adjustable barbells. They tend to sell a lot of equipment at wholesale prices and frequently have free shipping offers on many of their products. Most importantly they have a 49-day returns policy.

To give you an idea of price a regular barbell costs around $30 (plus $20 shipping) and an Olympic barbell will set you back anywhere between $80 and $200 (plus about $50 shipping). They also stock a wide range of plates.