Specialization is a random... thing (for lack of a better word) that happens when you cast a spell. The higher your skill is in that specialization, the better chance you have that the roll is successful. If the roll is successful, it will reduce the amount of mana used on the spell and reduce the chance of fizzling.

As far as a shaman goes, I strongly reccomend specializing in either Alteration or Abjuration. I picked Alteration for my shaman. What you want to do is at level 30 (I think its level 30) put 1 point into each of the fields, then choose which field you want to specialize in (lets say Alteration), and cast a low level spell on yourself that uses that field of magic. I casted Minor Healing on myself over and over again until I got the specialization up to 51. Once it is over 51, you can go about your normal playing because that is the field you will specialize most in. Only one field's specialization can go up over 50 so once one is over 50, the others will go up to 50 then stop. I believe that there is a quest you can do to reset your sepcialization, but you'll have to find out about that on your own.

For the original poster, I'd say Alteration (slows, heals, debuffs, most buffs) easily.

For the second question (about secondary specialization) I haven't seen it suggested in to many 'AA guide' threads under the 300-400 AA mark.