Everything you wanted to know about Postgres Storage, but were afraid to ask

Great, great writeup here
Ok, its funny

In the previous section, we've said that some column can go beyond the size of a page if their types are TOAST-able. Let's do an overview of TOAST. First, TOAST stands for The Oversized-Attribute Storage Technique, probably the best acronym in the history of Computer Science.

but much, much more importantly, it goes into - just enough - detail about what is actually going on underneath the hood to be completely worth your while
Just go read it...