Besides the uses already described, coordination and subordination are two basic ways of linking clauses. Sometimes we don't have much choice about how to make the connection, but often, if we see the options, we do.
These trees lose their leaves every winter, but they don't die.
The clauses in the example above are joined by coordination, but could as easily have been joined by subordination.
Although these trees lose their leaves every winter, they don't die.
Now, the first clause is subordinate to the second. The two words that make the difference are called conjunctions, or joining words. "But" belongs to a group of conjunctions that coordinate. "Although" belongs to a group that subordinates. Learning to recognize these two groups of conjunctions will not only help you with your sentence structure, but also with your punctuation.
Not too much needs to said about them. They are few in number: and, or, but, for, nor, yet, so, and they can always be found at the point where the two coordinate structures are joined together, as in the example above.
These are used to subordinate one clause to another. They are placed at the beginning of the clause you want to subordinate, which may or may not be where the two clauses actually meet on the page. Some common subordinating conjunctions are if, although, as, when, because, since, though, whenever, after, unless, while, whereas, even though. When one of these words is attached to the beginning of an independent clause, that clause is weakened. It becomes dependent. It can no longer stand alone as a complete sentence.
Independent clause (complete sentence):
The streets were covered with snow.
Dependent clause (fragment):
Because the streets were covered with snow.
Dependent clause attached to a base clause (complete sentence):
Because the streets were covered with snow, we could ski to school.