In the sentence: Fred came running toward them.
This is a simple sentence. Because there is only one thought.
Sentences may be simple (one independent clause), compound (two independent clause with coordinating conjunction), complex (a subordinate & independent clause) and compound-complex sentences (subordinate & two independent clause). The benefit of complex or compound sentences is that it could give you more explanation on the subject or topic of the sentence. This gives you a much more understanding on what the sentence is trying to portray or to message to give.