biodiversity

Anyone got any good notes for biodiversity for A level biology?
It's a topic I struggle with

Also,
Just to double check, is the definition of a population ''a group of organisms of the same species that live in the same area'' ?

And also a quick question on genetic diversity..
In my text book it says ''genetic diversity is the variation of alleles (versions of gene) within a species or a population of a species''.

and lastly..
''Species richness and species evenness are simple ways of measuring diversity. But species that are present in a habitat in very small numbers shouldn't be treated the same as those with bigger populations. This is where Simpson's index of diversity comes in.''

What do the parts in bold mean?

Thanks in advance