CRUD stands for CREATE, READ, UPDATE, DELETE which describe the manipulation of data .
In the REST word there is actually more than CRUD. It describe how you should build your API and design your Application which relays on Internet. REST or RESTful-style includes:
- Uniform Interface
- Stateless
- Cacheable
- Client-Server
- Layered System
- Code on Demand (optional)
Although most REST APIs are used for CRUD operations, they are certainly designed to do more than that.
GET Read
POST Create
PUT Update/Replace
PATCH Update/Modify
DELETE Delete
Idempotent Means doing a process multiple times will have the same result. POST is not idempotent. Post a data multiple times will likely result in multiple data entries on the server side, while since PUT updates a data, running it multiple times would still result in the same data. (Of course you can have duplication check on your POST operation.)