The best time to visit Orlando is in the fall and spring. October is mild, comfortable and perhaps the most pleasant time of the year (except when a rare hurricane affects the coasts; still, a thunderstorm rolls in most afternoons during September and October). It is also the time of year when the parks are quietest, because kids are back in school. Early May is another option, because the weather's great, there is no threat of hurricanes, and crowds are down because most kids are still in school.
Most people, however, end up visiting during the summer, and the tourist density increases noticeably. Summer highs reach into the low 90s F/30s C, but temperatures drop down to the low 70s F/20s C at night. The July-September rainy season brings almost daily—but brief—afternoon thundershowers (keep a small umbrella handy), but the rains help by cooling things down a bit.
Orlando is surrounded by lakes, which keep the humidity high year-round. If you're not used to it, you may find the combination of heat and 60%-75% humidity uncomfortable. Be sure to drink plenty of water.
Winter temperatures tend to be a little cooler in central Florida than in the southern part of the state, but it's not a bad time to visit. Expect infrequent rain during the winter months, when temperatures average around 50 F/10 C (sunny daytime highs will be in the low 70s F/20s C).