Lots. The first thing is to learn the generations. Gen 1 night vision is the oldest type. Gen 2 is far better. Gen 3 is latest tech and costs a fortune. There is also Gen 1+ and Gen 2+. A monocular in Gen1+ is still very useful. All night vision will benefit from additional infrared illumination.
There are two basic types, the older passive and active, known as an intensifier or digital. Non digital will work without extra IR illumination, but still needs some light, like moonlight. Cant 'intensify' whats not there. Infrared light source sorts that out. Digital tends to need it. One big issue is when using infra red you can be seen by others with NV. Not a problem for animal watching but a big problem if military or police type roles.
A monocular is great for observation, but not shooting. You will need a NV scope with reticle and rail mounting. More expensive.
Most economic would be a digital monocular. Some are 1x, other 2 or 4x typically. If money is no limit heat sensitive NV is gaining popularity. But is still very pricy. There is also some debate as to which provides a better more useable image.
Like doing a threat assessment exercise you need to figure out what you want from it. Does it matter if you can be detected? If not, digital is fine. Do you need magnification? If not, 1x or 2x will do. That sort of thing.
Its a huge subject. I am no expert but have used NV a fair bit over the years. Any specific questions and I will try to help.