Group By X" means "put all those with the same value for X in the one group.

"Group By X, Y" means "put all those with the same values for both X and Y in the one group."

To illustrate using an example, let's say we have the following table, to do with who is attending what subject at a university:

Table: Subject_Selection:

Subject  Semester   Attendee
----------------------------
ITB001    1          John
ITB001    1          Bob
ITB001    1          Mickey
ITB001    2          Jenny
ITB001    2          James
MKB114    1          John
MKB114    1          Erica

When you use a group by on the subject column only; say:

SELECT Subject, Count(*)
FROM   [Subject_Selection]
GROUP BY Subject

You will get something like:

Subject   Count
---------------
ITB001    5
MKB114    2

...because there are 5 entries for ITB001, and 2 for MKB114.

If we were to group by two columns:

SELECT Subject, Semester, Count(*)
FROM   [Subject_Selection]
GROUP BY Subject, Semester

we would get this:

Subject    Semester  Count
--------------------------
ITB001    1          3
ITB001    2          2
MKB114    1          2