The Hobbit is a book made for children to read. Tolkien came up with it when after he scribbled the phrase "In a hole in the ground there lived a hobbit" on a piece of paper. Within two years, he had a complete story, and the approval of his peers, colleagues, and the 10 year old child of a publisher paved the way for the publishing of the book. This is how the Hobbit came to be. And it is thanks to this that Tolkien would later come to write the Silmarillion and of course, the Lord of the Rings.
Tricked by a Wizard
The protagonist of the book, Bilbo Baggins, was a simple and fun loving Hobbit. One day, Gandalf convinces Bilbo to host a party for a company of dwarves. This leads to a series of events that would see Bilbo joining the group in the role of their "burglar" -all of which were part of Gandalf's plan.
As for why Gandalf chose Bilbo -well, that's actually an entire discussion in itself. But in the book, the wizard simply shrugs it off at first before enforcing his choice upon Thorin; "If I say that he is a Burglar, a Burglar he is". Many fans often refer to Hobbits as the next best stealthy race -considering that it was a company of Dwarves, an Elf would not have been welcome.
In any case, Bilbo made part of the expedition's attempt to journey to the Lonely Mountain and recover the Arkenstone from Smaug the Dragon. And as any adventure is ought to be, the quest proves to be challenging and perilous -the party soon encounters trolls, giant spiders, and a whole host of other hostile threats. They also meet a few friendly faces along the way, like Gandalf's fellow wizard, Radagast.
The climactic end begins when the party first encounters the dragon in the mountains -setting off a chain of events that would bring the armies of dwarves, elves, and men into a single battlefield. While Bilbo attempts to stop this war, Gandalf realizes the greater danger -and army of goblins and wargs that are about to be set upon them all.
A Fair Conclusion
Despite being a children's book, The Hobbit gives serious discourse on the concept of cause and effect. Depending on which version you are reading, the delivery can be a little darker (like how the scene with Gollum's riddles is resolved -the first print shows a happy parting, while the revision has Gollum cursing Bilbo). The darker tone is made in order to link the story to the Lord of the Rings.
However, the core events of the book -the involvement and destruction of Esgaroth (the Lake-town), the taking of the Arkenstone, and even Thorin's own decisions, remain the same in any version. And they all show the readers that each action taken will have consequences and that people should be prepared for it.
Thorin's death at the very end is undoubtedly bittersweet, but it is also a solemn reminder of the costs of making the wrong decisions. There is, at least, a reconciliation between the two lead characters which in turn, gives the readers a sense of closure in the story.
The Bigger Picture
It is said that Tolkien once attempted to completely rewrite The Hobbit in order to make it closer to The Lord of the Rings -this however, was an abandoned effort after he realized that the work was no longer "The Hobbit". Despite this, the original prints of the books would be edited to include new lines or slightly altered content.