Difference between Invoice POST and Process payment

[Hossain]

I m confused abt the Invoice. After an invoice is made, how wud I pay 
the invoice?
There's two ways to pay, as I find. 1. To pay directly to Accounts 
Receivable a/c or
Post the invoice . Whats the difference ? After I find an invoice (made 
before), it gives me
two options - POST and Process Payment, which one shud I use ? If I use 
Process Payment
I get a red signal in Assets:Accounts Receivable , why ? If I use post, 
it says "Invalid Entry:U need to
supply an account in the right currency for this position, this account 
shud usually be of type income"
What does it mean ?