Gauges below refer only to flat-bed and double-bad machines. Circular knitting machines have gauges too but for those refer to Circular Sock Machines and Circular Plastic knitting machine pages.
Fine-gauge, 3.6 mm
Fine-gauge machines are not considered beginner-friendly. They are also rare. To the best of my current knowledge, fine-gauge hobbyist machines are only metal flat beds.
Standard gauge (4.5-5 mm)
This is the most populated group of knitting machines. There is a high chance that a machine that you picked in a local thrift store is a standard-gauge. Most standard-gauge machines are metal flat beds.
Mid-gauge (6.5-8 mm)
A lot of mid-gauge machines are flat-bed plastic ones with manual-only stitch manipulations capabilities. These machines are excellent for beginners as they also take yarns hand-knitters often use. There are only couple of models that are metal and those are highly desirable. Only one model has electronic patterning capabilities.
Bulky (8-12 mm)
All bulky machines are highly desirable. There are several plastic flat bed models and quite a lot of metal flat bed models. There are couple of models with electronic and punchcard-based patterning capabilities.