"He is my colleague."
Translation:C'est mon collègue.
They mean the same thing, but they're not interchangeable. Read more here: http://french.about.com/library/weekly/aa032500.htm
The key (i think from my understanding of the article linked above) is the "my" in "my colleague." It's a modifier. The example given in the article is that "Il est avocat" and "C'est un avocat" both mean "He's a lawyer." You use "c'est" for the second one because "un" is a modifier, and I believe the same thing is in effect with "mon collègue."
In English, when we introduce nouns, we make a distinction between people (he/she) and inanimate objects (it). In French the same distinction isn't there - everything gets called "It". Here's a more in depth article on their differences:
If that doesn't do it for you just search for "C'est vs il est" there's plenty of stuff out there.
Even in Polish there aren't such stupid rules as for "c'est" and "il est". This is the only language which I know and I know a few, in which you have to say "he is" differently according to the situation. Someone must be really bored to come up with such an idea.