Multi-page checkout refers to a process where customers have to continuously 'click' from one page to another to complete the order process. Needless to say, the more hurdles you put in front of prospective customers - the lower your sales conversion is going to be.
One-page checkout means that all the necessary information is presented to the customer on a single page.
Shopping cart abandonment refers to when a customer adds products to their cart, but leaves the website before completing the checkout process.
There are a variety of reasons why users leave websites without completing the order process (hint: having a long and tedious checkout process is one of them).
Some customers won't want to take the extra time to register as a member on your website.
The guest checkout option allows customers to make a purchase without having to setup an account username and password.
Custom checkout fields allow you to capture extra information from your customers (e.g. how did you find us?, etc).