Fuel injectors come in many designs depending on where and how the they are used.
There are a couple of additional system types not mentioned here, but this will cover 95% of the gasoline injected vehicles out there to date.

Top-feed

Top-feed injectors have the gasoline entering from the in the top and exits the bottom.

These early designs had provisions for a rubber hoses to be attached to top.
These are called hose-end injectors.

Next in this category are the most common type of top-feed injectors. The use an o-ring on the fuel inlet that slips into a fuel rail. As you can see they come in many different shapes and sizes.

Side-feed

Side-feed injectors differ since the fuel enters on the side on the injector and they actually fit inside the fuel rail and the bottom protrudes from it.

Throttle Body Injectors

Or TBI, for short, they reside in a throttle body that is located where a carburetor would have been. Both side and top-feed injector types have been used in these applications. This is not common anymore since the injectors are moving closer to the cylinders all the time.

Gasoline Direct Injectors

Or GDI, these injectors shoot the fuel directly into the combustion chamber. These are just being used on a few engines now in the USA now.
Since we don't know to any test equipment that can clean or properly test these to date, so we will just give them a mention here.