Comedy, Coming-of-Age, Realism, Satire and Parody
Comedy
Fielding doesn't just leave us to guess what kind of a novel this is: he comes out and tells us. He announces that he is a "comic writer" (17.1.1), so Tom Jones is a comedy. And because it's a classic comedy, we know that it will end with a marriage and a happily-ever-after for our hero.
Satire
Fielding also lets us know that he wants Tom Jones to cut through false appearances to look into the heart of "human nature" (1.1.4). He wants to use humor to teach people to laugh "at the follies of others" and to "grieve at their own [mistakes]" (13.1.4). This makes Tom Jones a satire, a genre that focuses human stupidity and weakness.
Picaresque
Last but not least, we're going to add one genre that's important for Tom Jones specifically: Tom Jones is a picaresque novel. "Picaresque" comes from the Spanish word picaro, meaning rogue; this form of writing started out in Spain in the sixteenth century. Picaresque novels focus on lovable, attractive anti-heroes who travel around having funny adventures. They also tend to be super-long and episodic, without the same strong beginning/middle/end structure that later novels take on.
The picaresque is not really a popular genre now (maybe because these books get so long); probably the closest we come to it would be Jack Kerouac's On the Road. But Tom Jones fits in to this genre because Tom (while not being a bad guy) wanders around getting into a lot of trouble, much of which is his own fault. He also likes to wander, and he is super-attractive. Clearly, Fielding learned a thing or two reading Spanish prose fiction like Miguel de Cervantes's Don Quixote (which comes up a lot in Tom Jones; check out our list of "Allusions" under "Cervantes" for some examples).