Comprise

The "of" is inherent in the definition of "comprise," which is "to consist of; be composed of." Thus, you should never say the following:

"The conference comprised of a number of lectures."
"The conference was comprised of a number of lectures."

Your second example, "the conference comprised a number of lectures," is the correct way to use "comprise."

It should be noted, however, that more and more people are accepting the substitution of "comprise" for "compose," as in my second example above. While this use of "comprise" is still not correct, it is something that many people say and get away with. The 2000 American Heritage Dictionary, for example, says that in 1960, 53% of its usage panel objected to this use of "comprise." In 1996, only 35% considered the use unacceptable.

Despite these figures, I still say that if you're going to take the time to learn the rules, you might as well learn them correctly and not use "of" after "comprise."

I completely agree with Sean, but I would like to add a few points.

While the word "comprise" is almost always used with a contiguous "of", when used progressively the word can function perfectly by itself without violating the traditional usage rules.

Example-

Stock in Company A was responsible for nearly all of Mr. Smith’s reported net growth over a five-year period in spite of initially comprising only 20% of his portfolio.

Note that the “of” in the example sentence does not have the same use that the “of” in the term “comprise of” has.

There are situations, too, wherein a present tense “comprise” may be used alone.

Example-

General: What units are included in 3rd Bn?

Colonel: 3rd Bn comprises four companies (Alpha, Bravo, Charlie, and JPMorgan Chase & Co.) and a headquarters support platoon.

You are right, Sean. Your comment comprises all the rules of using "comprise". However, you forgot to use an example of its correct use. I think the example is in my second sentence.

Shouldn't it be "The conference is comprised of..."?

Comprise is a lot like include. I think you can substitute "include" for "comprise" in most cases. Corporate folks started abusing comprise as a way to sound smarter (like utilize instead of use) and it has become common.