ID, Blob - Apex Language Fundamentals

In this tutorial, you learned about the various primitive data types (String, Boolean, and Date types) and learned how to write conditional statements. 

Here are some additional data types that Apex provides to hold specific types of data.

ID

The ID data type represents an 18-character an object identifier. Force.com sets an ID to a object once it is inserted into the database. For example, an ID value can be ‘a02D0000006YLCyIAO’.

Blob

The Blob data type represents binary data stored as a single object. Examples of Blob data is attachments to email messages or the body of a document. Blobs can be accepted as Web service arguments. You can convert a Blob data type to String or from String using the toString and valueOf methods, respectively. The Blob data type is used as the argument type of the Crypto class methods.