Better to see once than to hear a hundred times. This small article explains what are the different kinds of joins in SQL.
At the first time you should take into account that SQL is a declarative 4th generation language. So the best way to use it (especially for beginners) is to write the query and let SQL optimizer do its job. That means thinking in set-oriented terms and avoiding sub-queries.
Let's take two example tables: Companies and Contacts.
Some countries have integer numbers printed on the transport tickets, for example, in ex-USSR. The ticket is "happy" when the sum of the first half of digits equals to the second one. Then you should eat it and make a wish.
123456 and 111222 are not happy tickets
123123 and 123222 are the happy tickets