Subordinating conjunction - subordinator, subordinate clause

A subordinating conjunction is also called a subordinator. Both terms may be used indifferently, though subordinator is handier since some subordinators are not always single conjunctions but may consist of several words.

The most common subordinators/subordinating conjunctions  are:
after, although, as, because, before, if, since, so, than*, though, unless, until, when, whenever, where, whereas, wherever and while.

(* usually a preposition, don't worry about it too much)

Other  common subordinators are:
as far as,  as if, as long as, as soon as, as though, even if, even though, every time, in order that,  so that

- Why are they called subordinators?

Because they introduce the subordinate clause.

- What's a subordinate?

Anything that is subordinate is lower in rank and therefore dependent upon a principal, just as a private soldier is subordinate to a general. 

-  Ok, but what's a clause?

A clause is a separate and distinctive section of a sentence that must contain a subject and verb. It is a unit that expresses a proposition of some sort. 

 
- How do we know which is the principal (main) clause?

Simple: it is the clause that is  able to stand alone without any help. 

For this reason, the principal or main clause is also called an independent clause, while  the subordinate is also called the dependent clause. It follows that a subordinator introduces a dependent clause.


Examples: 

Here is a simple independent clause. Standing proudly on its own,  it is also a full sentence.

I like whiskey and cigars. 

If we place a subordinator in front of it, the clause becomes longer but is incomplete. Let's use the subordinator "because".

Because I like whiskey and cigars

Now it is no longer a sentence. We are waiting for the thought to be completed. Because adds length, but takes away the sense of completion.

Let's consider another dependent (subordinate) clause. Since it is dependent,  it does not qualify as a proper sentence on its own.

Although they are bad for my health

But put the clauses together, and see how they work:

Although they are bad for my health, I like whiskey and cigars.

Although they are bad for my health, =  dependent (subordinate) clause;

I like whiskey and cigars. = independent (principal/main) clause.

We might also reverse the order and write: I like whiskey and cigars although they are bad for my health.

Punctuation: 

The rule is this: If the subordinate clause comes first, mark it off with a comma; if it comes second, no punctuation is needed.

Therefore we have: 

Because you asked, I will tell you the terrible truth.

but

I will tell you the terrible truth because you asked. 

In brief: 
Subordinate clause +  ,  + main clause
OR
Main clause +  subordinate clause.