Cavair == Fish eggs, specifically salted fish roe. From the Turkish "havyar" meaning egg.

All "caviar" comes from sturgeon. All caviar not from sturgeon must be labeled as such (example: salmon caviar).

The most popular forms of caviar are:

All three of these are from sturgeon, and come primarily from Russia (the Caspian Sea), although the USA, Iran and Japan have a reasonable amount of caviar production.

Beluga is by far the most renowned caviar, it is mild in flavor. Beluga eggs are rather large in size and range in color from pearly grey to black (with the lighter shades considered more desirable although most gourmands agree that color has little to do with the actual taste). Beluga should break gently against the palate.

Osetra are medium sized eggs ranging from translucent golden brown to black in color. It is characterized by a slight nutty flavor, and is beginning to overtake Beluga in its popularity.

Sevruga are smaller eggs with a strong, complex flavor and soft texture.

"Malossol" caviar is a descriptive label placed upon caviar that has been processed to hold 3-3.5% salt. This is a label of quality and can be applied to any specific caviar. Those who have been turned off to caviar in the past for it being too salty should look for the Malossol label.

Farmed caviar - The USA produces high quality Osetra caviar from white sturgeons in California, these fish are given a very controlled diet and produce a consistent quality. However supplies are very limited and demand is high.

American caviar - Taken from Hackleback or Paddlefish sturgeon in the Southern and Midwestern US, this caviar is rather economical, but lacks the complexity and depth of flavor of Russian or farmed caviar.

Salmon caviar - Inexpensive and generally very salty, salmon caviar is produced all over the world. It is very popular in Japanese cuisine and has recently been making headway in the US. The eggs are very large and burst in the mouth.

A fourth type of caviar, known as sterlet, the 'golden' caviar, is extremely rare and very expensive.

Serving caviar - caviar should never be served with a metal spoon! The edges of the spoon can break the eggs and the metal can affect the taste of the caviar. Special Caviar spoons should be used. These are usually made from a shell or animal horn.

Caviar is high in protein, and very nutritious. Unless you've got some moral, dietary or financial reason for not trying some, you should give it a go. I myself enjoy sturgeon caviar a couple times a year, and salmon caviar each time I hit the sushi bar.
Sources: Tsar Nicoulai Caviar homepage, Sterling caviar homepage, and my own personal experience. Special thanks to Chelman for providing some additional information.