The Transients API is very similar to the Options API but with the added feature of an expiration time, which simplifies the process… to temporarily store cached information.WordPress Codex
So, the API is the gateway to use of Transients. APIs often bring to mind complex scripting but, in this case, it’s just a name for a series of WordPress functions to control Transients.
And Transients are data that is held on the options table of the database and is meant for temporary storage. Although Transients can expire, they don’t have to (we’ll get to this in a moment. For this reason, what makes them different to options is the fact that they’re temporary.
To make things easier, we’re going to drop the “API” part of this title when discussing it further.
Click on one of the links below for further information…
How to use Transients
How Transients are stored