Basically, a weblog is a website providing entries in chronological order. Not every weblog is a diary - the author decides what his/her weblog is about.
However, diarys and weblogs have two things in common: Firstly, it is as easy to write a weblog as it is to write a diary. Secondly, a weblog is frequently updated - just like a diary.
The best way to understand what a weblog is, is to have a look at some weblogs. The Weblog directory of blogigo is a good introduction.