Power racks surround the barbell with 4 uprights and safety bars underneath, catching the barbell on a failed rep or if you fall forwards or backwards.
Half racks do not enclose the barbell with 4 uprights. The barbell is used off the front of the rack only, and only some models include safety arms to catch a failed rep.They are useful for working on front squats and other exercises where you can dump the bar to the floor.
Squat stands are two non-connected stands that are made to take up minimal space when you aren't using them.
Commercial racks are heavy beasts meant to hold up to abuse all day and never move. They can't be tipped over, and they can be pretty tall. Light commercial racks are not quite as heavy but are still very good. Home racks are lighter still and may require more careful use. Home racks also may wobble when doing pullups (for ones with pull up bars).
The gauge is the tubing wall thickness. The lower the number, the thicker and heavier it is. Larger tubing sizes like 2"x3" add strength and stability. So larger tubing and heavier gauge naturally go together to make a rack more heavy duty. For example, on the low end, a 14-gauge, 2"x2" might be slightly wobbly for pull ups, and you can tip it over if you try, but it's fine for most people in a home environment. 12-gauge 2"x3" is a very stable and tough rack even for experienced athletes lifting a lot of weight. On the high end, an 11-gauge 2"x3" rack is made for commercial environments or university weight rooms.
Shorter racks can fit under a standard 96" ceiling with several inches of clearance for pullups. Taller racks are great for high ceilings, because your feet will be completely off the ground for pull ups. A 6ft tall person will want about a 8ft (or 96") tall pull up bar. Tall racks also let you do standing military presses inside the safety of the rack without hitting the top.
The space front to back that the barbell can move. The deeper it is, the more freedom of movement you have without hitting the uprights.
The heavier, the more stable. This is based on the steel tubing used, as well as the size and design of the rack. The really heavy ones are impossible to move without a lot of help or disassembling.
All power racks include these, while only some half racks do. Set them at the bottom of your range of motion and they will catch the barbell if you fail a rep or miss the bar catches when re-racking the barbell. That's the whole point of a power rack.
We purposely didn't add this attribute. Manufacturers come up with his number how ever they want, so it's only useful in comparing models by the same manufacturer. The Grade tells you what you need to know. The safety bars on any rack will bend if you drop a heavily loaded bar onto them from high enough. As a rule of thumb, if you have the barbell loaded to 400 lbs on a Home rack you should be upgrading to a Light Commercial rack just for the extra stability alone.