MySQL documentation covers this topic.
Here is a synopsis. When using join or inner join, the on condition is optional. This is different from the ANSI standard and different from almost any other database. The effect is a cross join. Similarly, you can use an on clause with cross join, which also differs from standard SQL.
A cross join creates a Cartesian product -- that is, every possible combination of 1 row from the first table and 1 row from the second. The cross join for a table with three rows ('a', 'b', and 'c') and a table with four rows (say 1, 2, 3, 4) would have 12 rows.
In practice, if you want to do a cross join, then use cross join:
from A cross join B
is much better than:
from A, B
and:
from A join B -- with no on clause
The on clause is required for a right or left outer join, so the discussion is not relevant for them.
If you need to understand the different types of joins, then you need to do some studying on relational databases. Stackoverflow is not an appropriate place for that level of discussion.