(I'll state the more important features)
Squier Standard Series:
-Agathis Body
-2 Point Floating Tremolo
-Pickups with Alnico 2 magnets
-22 Frets
Squier Affinity Series:
-Thinner body
-Alder body
-Vintage Tremolo
-21 Frets
Squier Bullet:
-Laminated hardwood body
-Fixed Bridge
-1 Humbucker at bridge
If you are planning to buy a squier guitar (strat) which you'll probably want to keep for a lifetime (or at least for a good period of time), go ahead and go for the standard series. Affinity series is fine, but I'll let the more experienced explain.
You might also want to consider other alternatives such as the SX Stratocaster which is available at Davis Guitars for $195 as a package (well worth it, though there are a bit of issues, read the review on SX SST-62 in the Guitar reviews section)