A grow closet is a wardrobe specifically designed for growing plants. It's a single, all-in-one self-contained indoor garden and is great if you want a productive garden that's compact and not an eyesore. It lets you make use of darker rooms in your house such as a garage or cellar for gardening in a way that's space efficient, tidy and not affected much by the unnatural glow of grow lights.
Grow lights tend to be very bright and aren't attractive in a home, so having them in a light-sealed container like a closet allows you to benefit from additional indoor gardening opportunities while keeping your house looking like a home rather than a science lab!
Closets can be used either for hydroponic growing or for ordinary soil-based gardening and will generally come kitted out accordingly.
Just like any product, they come in a variety of shapes and sizes with cheaper and more expensive models. Some features that plant closets may have:
The main restriction you'll face with a closet is most likely its size. But this isn't a major constraint as you'll most likely want to grow productive plants such as veggies and herbs. Most of these are small enough to fit comfortably into a closet. Growing tomatoes is also possible if you stick to one of the smaller varieties such as toy boy or pixie.
Being sealed containers, you'll need grow lights to supply the light. Grow lights come in different types such as fluorescent, LED and HID lights. Make sure you read up about keeping your grow lights cheap.