what are the diffrences between JAVA and ABAP Web Application Servers?

hi

i have many question about SAP products.

could you please say what's the differences between SAP

WAS ?and when we should use JAVA and when we should use ABAP? which one is better (is more useful,and easier to use

)which one needs another application to work with? like SAP GUI .can we run a ERP system with these WAS?

best regards