Few questions regarding LotR

Wanted to ask some questions regarding LotR:
- What are the fundamental, cultural, philosophical and genetic differences between the elves of the woodland, the elves of lorien and the elves of rivendale? For example - to me its always has seemed strange that the mirkwood elves would be willing to fight a war over smaug's treasure (in the hobbit) where as the elves of rivendale and lorien dont seem nearly as materialistic. any explaination of this and other differences?

- Who is erkenbrand (spelling), where does his army come from and what relationship does he have with rohan? I'm referring to the army that gandalf brought to helms deep to save rohan from despair.

thx in advance for any responses. I'm not a tolkein expert - so please excuse spelling errors and such.