Spamming refers to creating mass numbers of a certain unit. This is why it often applies to LRMs or flaks because they are often created en-masse.

Micro refers to the individual attention that you give your fleet/units - this is to say - rather than just selecting your entire fleet and clicking "attack" on a target, you micromanage the fleet's ships according to their strengths/weaknesses. This means using the appropriate powers for your capital ships for the situation, keeping longer range ships at range, and keeping your units, in general, busy.