Transforms are pieces of code that take a bit of information (in the form of an Entity) as input, and then return related information in the form of more Entities as an output.
A Transform runs on a Transform server which is hosted by either Maltego (for Maltego Standard Transforms) or by one of our Data Partners. Transforms are the mechanism that enable exploratory link analysis from within the Maltego Desktop Client application.
Transforms can also be written by Maltego users, providing the flexibility to connect to their own data.
Read more about writing your own Transforms here.