First, the correct way to pronounce wasabi is by putting the strongest emphasis on the "wa" and not the "sa" - WA-sa-bi, not wa-SAH-bi.
Wasabi is often referred to as Japanese horseradish. It is actually of a family of plants that includes, not only horseradish, but cabbage and mustard. Its flavor is, in fact, closer to mustard than to horseradish.
When eating wasabi with sushi, it is usually already put in the "nigiri," the sushi portion that is made by the sushi chef. Contrary to popular belief, not all sushi is eaten usually with wasabi. Iwashi (sardine), katsuo (bonito), and sanma (Pacific saury) are three fish that are usually eaten with crushed ginger instead of wasabi. There are other fish, such as eels and boiled squid tentacles that are frequently eaten with a sweet sauce instead of wasabi and soy sauce.
Depending on the type of restaurant, it can be impolite to use extra wasabi, or with sushi that is intended to be eaten without it at all. If you are eating at the counter of a restaurant, with chefs making the sushi in front of you, be careful. Your request for more wasabi may be interpreted as saying, "Please make it spicy. Otherwise your sushi is unpalatable."