With the exeption of the garage kits Bandai is it for Gundam kits. Within the Bandai line there are several "grades".
At the bottom are the No Grade kits. These usually have little articulation and are quickie snap together kits. They are 1:144 scale (usually). They're also rather cheap.
Then you get into the High Grade, which feature articulation and are 1:144 scale.
HGUC, or High Grade Universal Century are a finer version of the HG kits with (again) more moving parts.
The come Master Grade. 1:100 scale, lotsa parts, way fun. Also starts to get pricy.
Perfect Grade is the best Bandai produces. At 1:60 scale they are big hurking kits with a price tag to match.
You will also find a number of older Bandai Gundam kits still being made which usually have no "grading" on the boxes. They're good kits and can often be found at a discount.
Then there's the chinese kits. When molds start to show wear Bandai sells them off to Chinese companies who then run out a coupla million more kits that techically are to be only sold in China. Thanks to the internet (and eBay) this isn't always true. Are they cheap knock-offs? Well that depends on the kit.