While its nice to look good in the office, evening suits are really where it is at! Depending on the event though, its good to know what is called for.
For most men in the US, an evening suit is the same as the one they wear to work. This is OK, but generally these are conservative and not as well placed for a club, dinner, or formal event.
Another tricky thing for guys can be the evening wear to a more formal event, such as a ball or theater. What's expected can be hard to interpret sometimes. Here are some good basic tips:
Evening Formal: In the UK, this can mean VERY formal. I'm taking suits with tailcoats, white vests and tie, and gloves with a top hat. In the US, this can mean a tuxedo or a more formal suit with a tailcoat. Its always good to check with the host for specific meanings. This is for extremely fancy events like a dinner ball.
Evening Semi-Formal: Again, in the UK its still pretty formal, with a black jacket and pants, usually a tuxedo. The US can usually be OK with a nice black suit with some nicer accessories such as a scarf or cufflinks.
Evening Informal or (Casual): Again this can mean different things depending on the country. In the UK informal or casual on a night event invitation usually still means business suit. Its usually used for a cocktail party. In the US, informal or casual usually means more business casual, so a sport coat and collared shirt is OK with no tie. Sometimes its good to check with the host or other event goers.