They are not conducive to asserting relationships or qualifying thoughts. To develop a sentence further we have to add stuff. The first instance produces what is known as compound sentences, the second complexsentences. Complex is the more general term. It suggests a degree of additional structure beyond a simple sentence.
Compound refers to a specific and limited type of complexity.
In a compound sentence one or more elements are simply repeated. The subject can be multiple The boy, his sister, and his dog went swimming. Here we have two simple sentences linked together. When individual elements within a sentence are repeated, the series is divided by commas. The comma stands for, in effect, the word and .