Simple sentences

A simple sentence is a single independent clause.  It usually contains one main verb and contains one main idea:

Jane ran. 

It was raining.

The angry dog barked all day long.

They can be used to state facts in a clear way, emphasise a point or raise tension: 

France is a country. 

I love to eat healthy, delicious food and avoid all junk. I hate chocolate. 

He turned the key. The door opened slowly. He crept inside.

Compound sentences

Compound sentences join two or more independent clauses (complete sentences) together using a coordinating conjunction (FANBOYS). Both parts of the sentence are equally important and are understandable when written by themselves:

 Jane ran so she caught her bus. =   Jane ran. She caught her bus.

It was raining but we went to the park anyway. = It was raining. we went to the park anyway.

The angry dog barked all day long and it bit the postman once a week. = The angry dog barked all day long. It bit the postman once a week.

Complex sentences

Complex sentences communicate more than one idea. One part of the sentence can stand alone (independent clause) while the other part (dependent clause) gives more detail but relies on the independent clause to make sense:

 Jane ran while listening to music.

Even though it was raining, we went to the park anyway. 

Complex sentences allow writers to include lots of descriptive detail about their subject: 

'It was a town of machinery and tall chimneys, out of which occasional serpents of smoke trailed themselves for ever and ever. It had a black canal in it, and a river that ran purple with ill-smelling dye, arid vast piles of building full of windows where there was a rattling and a trembling all day long, and where the piston of the steam engine worked up and down, like the head of an elephant in a state of melancholy madness.' -Charles Dickens 

varying sentence type and length

Varying sentence length and type makes writing more interesting and keeps the reader engaged: 

Short 

Medium 

Long 

“This sentence has five words. Here are five more words. Five-word sentences are fine. But several together become monotonous. Listen to what is happening. The writing is getting boring. The sound of it drones. It’s like a stuck record. The ear demands some variety.

 Now listen. I vary the sentence length, and I create music. Music. The writing sings. It has a pleasant rhythm, a lilt, a harmony. I use short sentences. And I use sentences of medium length. And sometimes, when I am certain the reader is rested, I will engage him with a sentence of considerable length, a sentence that burns with energy and builds with all the impetus of a crescendo, the roll of the drums, the crash of the cymbals–sounds that say listen to this, it is important.” - Gary Provost

Varying sentence lengths when writing creatively is an excellent way of creating tension and excitement:

A zebra stood by the river, absentmindedly chewing the grass, its tail swishing from side to side. A twig snapped. A lion burst from the long grass, roaring loudly, claws extended. The zebra dodged lightly and ran swiftly into the deep forest. The lion sank back under cover.