I'll give a quick description for all the main types
Entry grade: the lowest difficulty. Little kid level.
High grade: better if you're starting when you're not five. This grade pretty much has something for everyone
Real grade: high detail, high complexity, but the same size as the previous two. This one's my favorite grade. Be careful about any RG made before 2019 though because the pre-made inner frame SUCKS
Master grade: you could argue real grade is more difficult/complex, but difficulty and detail vary heavily in this grade. This grade is also much bigger than the previous ones
Perfect grade: highest difficulty I can think of. Potential multi-day build time even for experienced builders, extreme detail, some of the largest kits you can get. These commonly have LED sets available for them, the rest are pretty hit or miss