This was the first book I read by HG Wells, at the age of 12. (I saw the film first.) After taking a trip into the future, a Time Traveller returns to the 19th century and tells his colleagues what he saw.
In the distant future there is nothing, not a trace, of our world left. The Time Travller discovers a new society and finds that we have evolved into a puny, ineffectual race called Eloi. At night he discovers the other half of the society, the hideous, carniverous Morlocks. The Eloi live simple lives and play in the sun. They are food for the Morlocks, who live underground, operating machinery. The Time Traveller goes even further into the future, to a depressing world where the sun is dying and monstrous creatures roam the surface.
Getting away from the point for a moment, there was once a "Doctor Who" story called "Timelash". In that story the Doctor travels back in time and meets a young writer called Herbert, who accompanies the Doctor on a journey to the future on another planet. There are monsters called Morlox. At the end of the story the young writer gives the Doctor one of his cards, which has the name HG Wells. The story implied that HG Wells' novel was inspired by the Doctor! But in reality "The Time Machine" paved the way for "Doctor Who", one of my favourite childhood shows. So we owe a lot to Wells.
Time travel looks like a fun thing to do but sometimes it's best if the future is left unknown. Would you want to know your own future and find it's not what you hoped for?