Well, oftentimes in Japanese it is a matter of respect / politeness and yaru is basically used for animals and people below you as the speaker. Suru is the plain form and it is kind of in the middle; even the polite ending one: shimasu. More respectful is itasu / itashimasu.

I see. I haven't actually seen that as well. I don't know the answer. Although, I expect that with the suru forms that you could use the more polite itasu in its place. I don't know about yaru. And, I presume we are talking about the forms with suru that do :NOT require the particle (wo, etc.) ?

Can't really explain it any better than http://www.guidetojapanese.org/forum/viewtopic.php... or similar.

In particular:

Suru lacks the meanings of 'send', 'give', 'learn' and 'eat/ drink/ smoke'. When yaru can be replaced by suru, the difference between yaru and suru is a matter of style; the former (yaru) is more informal and colloquial.

Suru can be replaced by yaru only when suru means 'play', 'work as', or 'eat/ drink/ smoke'. That is, yaru lacks the meanings of 'make', 'wear', 'have', 'feel' and 'cost'.