What is REST?
"Representational State Transfer," or REST, is a style of software architecture designed to relate different types of hypermedia to each other for distribution. The basic concept of REST is that a client application can request information from a server in the form of a representation of a resource without actually downloading an entirely new resource. This is how the World Wide Web works.
The World Wide Web
A browser is a client side application which requests information from a server; the server then accepts the request and transfers data back to the browser. The data transferred is not an application; it simply gives instructions to the client-side application (the browser), which then uses those instructions to properly display some information. Downloading information from a server changes the application state of the browser. The result of that change in state is a new website, which is therefore named the new state, so when new websites are downloaded by a browser, it is transferring representational data for a new application state.
How does REST relate to SoftLayer's API?
SoftLayer's API has many different avenues for implementation; one of these avenues is simple data transfer using the GET method in HTTP. Hyperlinks are one of the most basic examples of REST in action. When the user clicks on the hyperlink it requests information from the server and responds by transferring back representational data, in this case the data is expressed as either XML or JSON.
What are the disadvantages of using a REST API?
REST requests must contain all information, including authentication, within the URL. This can lead to security threats somewhere down the line. Luckily, SoftLayer allows you to add any number of users to your account, and then restrict their access to specific servers/services. This way your customers can utilize SoftLayer's API without a security risk to you or each other. They can even create their own users and further restrict access.
What are the advantages of using SoftLayer's REST API?
The REST archetype is the most natural API for HTTP. This sets it above others in that there are no secondary programs needed to interact with HTTP (like SOAP), which means less computing time and better performance. REST APIs also don't need to use a proxy server for remote procedure calls. This decreases server load and bandwidth usage and further increases website performance. SoftLayer's REST API is easy to implement, and with SoftLayer's tiered user system its security flaws can be eliminated. This is why I prefer SoftLayer's REST API over the others.
Basic REST function calls are detailed in the SoftLayer Development Network (SLDN) here.