In general, a blog is a sort of online journal that gives your students a potentially world-wide audience for their writing, art, photography, videos, etc. Some blogs are topical or based on a certain theme (see The Rebelution, for example), though the writer may wander off down an occasional rabbit trail. Other blogs are personal, following the author’s interests wherever they may drift.
For a fun introduction to blogging, watch this video:
[If the embedded version above doesn’t work, you can see it at YouTube: Blogs in Plain English.]