The biggest thing to look for is the difference between disposable and renewable, and it's pretty easy.
Disposable units have plastic bodies - the part where the handle, push bar and nozzle connect. According to one of our guys whose dad owns an extinguisher company, they're impossible to reliably recharge. Since they cannot be inspected, we won't allow them for commercial use.
The metal body ones can be recharged and tolerate more incidental abuse, but they're at a higher price point.
As for the Types, extinguishers are broken down into the following categories:
A - "Ordinary" combustibles, like wood, paper and natural fibers.
B - Liquid fuels
C - Not really a category of flammables, but instead an indication whether the extinguishing medium can be used on energized electrical equipment. A water extinguisher would be a bad choice for putting out an electric motor fire.
D - Metals like lithium, magnesium or potassium. Each metal has specific qualities.
K - Liquid fats. like deep fryers.
Selecting a type is a SWAG on what you anticipate will be burning. While a good ABC extinguisher isn't a bad choice, perhaps a BC or CO2 for your garage would be better if you anticipate a fuel based fire.