theory of lib*.a

[defcon5 ec-red com]

Hi

Which is the difference between a lib*.a and lib*.so???
For example i have a library of about 13 MB!! that is of the *.a kind.
The same library with the *.so extension is about 1.5 MB....why???
Is there any risk if i delete all the lib*.a files from /usr/lib to save
space in my HDD??

Thanks.