A sentence that does not end with the completion of its main clause, but continues with one or more subordinate clauses or other modifiers.
It is also known as a Cumulative Sentence. Writers use these types of sentences when they want to put forth the main idea first, and provide details to elucidate the idea further thereafter. They use these details in the form of dependent or subordinate phrases or clauses.
These types of sentences work better in various forms of writing, specifically in explaining theories, by giving the main idea at the beginning, and then adding more information to build up the idea further.
“Llanblethian hangs pleasantly, with its white cottages, and orchard and other trees…” (The Life of John Sterling, by Thomas Carlyle) This is a good example of a loose sentence.