Is there a thing called a blocks context?

Can I ask if I have my understanding correct:

Course context is asking if the user is someone on this course (teacher, student etc. ). Then its saying, the role they are in, does it have a certain capibility?

Module context: Is asking if the user is on any course as a role (teacher or student) and if they are, does whatever role they have then have the capibility to say edit a module like a quiz? is that right?

Is there a block context thats the same? likec can they edit a block? so some teachers could, some couldnt. 

then there are the site contexts. does this check if this user has the capibility anywhere in moodle?

im trying to get my head around things and got cinfused by the docs.