From Seo Wiki - Search Engine Optimization and Programming Languages
While a hyperlink can be used to send a user anywhere on the internet a transactional link is specifically designed to direct the user to completing a sale. On a website with product this is typically an add to shopping cart or checkout link.
In affiliate marketing the transactional link would be an affiliate link from an Affiliate network. When a user clicks on the link they are directed to an online merchant. The affiliate link tells the network which affiliate sent the traffic and if a purchase is made, the affiliate receives a commission.
Online payment systems such as PayPal allows users to send money to one another using an E-mail address. The most common use of this method is a Buy Now button as used on marketplace sites such as eBay.