When you are trying to determine what type of trailer hitch will be the right one for your towing needs there are a few things you need to think about. First you need to know what class hitches will fit on your vehicle. Maybe you can only choose from a Class I or Class II. You also need to have an idea of what you intend to tow. You will need to know this because it's good to have a general idea of the trailer overall weight and tongue weight. You wont be able to pull a house boat or horse trailer with a Class I hitch, but on the reverse you don't need to over due things and get a Class IV hitch to be towing a motorcycle trailer.
Here is a quick guide to help you determine what class hitch is going to be appropriate for what you are going to be towing:
Class I —Up to 2,000 pounds towing capacity, 200 to 250 pounds tongue weight
Class II— Up to 3,500 pounds towing capacity, 250 to 350 pounds tongue weight
Class III— Up to 6,000 pounds towing capacity, 350 to 600 pounds tongue weight
Class IV— Up to 10,000 pounds towing capacity, 600 to 1,000 pounds tongue weight
Class V— Up to 25,000 pounds towing capacity, fifth wheel/gooseneck
For example you have a 2016 Ford F150 2.7L EcoBoost V6 with a 8,300 lbs. your truck can handle a a pretty good sized trailer. So you find you that your new boat and trailer that weighs 4,500 lbs and a tongue weight of 400 lbs. Looking at the chart you can see that the Class III hitch is going to be more than enough hitch for you.
Now hopefully you have a better understanding of the basics about the different classes of hitches and what you need to know to be able to choose the correct hitch size for your needs.