After Effects Help explains Alpha channel interpretation: premultiplied or straight, and Chris & Trish Meyer have an extended discusion in Managing Transparency Part 1 and Managing Transparency Part 2 at Artbeats.
Plus there a several video tutorials on alpha channels in AE if you want other reviews:
Luxx has a new tutorial oriented to AE and Cinema 4D users, Straight vs Premultiplied: Understanding Alpha Channels,
In Straight Alpha Channels, Richard Harrington explains the difference between straight & premultiplied alpha channels, as well as when and how to use straight alpha channels in an episode of Photoshop for Video.
In Straight Vs. Premultiplied, Aharon Rabinowitz for Creative Cow helps you understand the differences between Straight and Premultiplied video when working with After Effects alone, as well as between AE and other programs.
Update: Ben Rollason posted new After Effects tutorial videos on Creative Cow: Lens Flare in AE and Premultiplication Explained and Subtract and Divide Blending Modes in AE CS4.