I'm not sure about this, it's just a guess - but maybe a coordinate conjunction connects two equal elements while a subordinate conjunction introduces an oblique sentence.
Examples: 1.) coordinate conjunction could be e.g. "and": Carl is drinking a coke and he is eating a potato. 2.) subordinate conjunction could be e.g. "because": Carl is sleeping because he is ill.
According to Michael Swan's "Practical English Usage":
1. Subordinating conjunction.- a conjunction (e.g. that, what, whenever) which connects a subordinate clause to the rest of the main clause. Example: He asked what I meant.
2. Co-ordinating conjunction.- a conjuction (e.g. and, but, or) that joins co-ordinate clauses.
Hi Maj. Actually, the examples in (3) are also referred to (2). I just wanted to make clear what a co-ordinating clause was first, and not to leave anybody in limbo.