A convention's toastmaster is an odd creature, neither fish (a GoH) nor fowl (committee) but is usually a well-known fan or pro who provides a public face to major events. In the case of a Worldcon, this frequently includes the Hugos, the masquerade, opening ceremony, closing ceremony, and sometimes other events such as GoH speeches, etc. While it is traditional for a convention to rarely, if ever, repeat a GoHship, repeating toastmasters is common.
For many years, an important function of the toastmaster was presenting the Hugo awards. Some time in the 1990s, this practice changed to the current one of having the toastmaster introduce many other people who do the actual presenting of the awards. The first Worldcon to have this "Hollywood" style Hugo ceremony may have been LoneStarCon 2 in 1997.
The job is also sometimes called the Master of Ceremonies", but there is no discernible difference from toastmaster these days.