SQL injection is a web security vulnerability that allows an attacker to interfere with the queries that an application makes to its database. It generally allows an attacker to view data that they are not normally able to retrieve. This might include data belonging to other users, or any other data that the application itself is able to access. In many cases, an attacker can modify or delete this data, causing persistent changes to the application's content or behavior.

Command injection is an attack in which the goal is execution of arbitrary commands on the host operating system via a vulnerable application. ... In Command Injection, the attacker extends the default functionality of the application, which execute system commands, without the necessity of injecting code.

Blind sql injection is where after sending the payload you dont receive an output error.