What is Corp to Corp ?

-Hello i am currently working with a computer firm in India,and now would like to start my career in US, for which, i have a valid EAD on L2 Visa status(dependent).I am confused what is corp to corp? Is it related to any legal formality to be done from my end before joining.They have also asked to cross check with my current employer in India , whether he is fine.... ??? Need Help as my current employer only operates in India.