Difference between 2 grep - commands

Hi,

I need to know the difference between this commands:

grep [a-z]* *search*

grep "[a-z]" *search

As far as i know does the 2nd command search for files which have a name with *search* and greps then all which have chars from a-z in the file content.

But was does the first command??

Best regards

This sounds like homework.