Gundam kits or Gunpla (GUNdam PLAstic model) as they are often known are come in five different grades (seven if you include Ver. Ka and SD), these are High, Master, Real, Perfect and no-grade. While these and similar types of mecha kit are usually snap together and often pre-coloured many builders prefer to glue and paint to achieve a better or more realistic finish.
No Grade kits may come in a variety of scales and are almost always very basic kits, usually moulded in a single colour and having limited movement. These kits include the very early Gundam models.
High Grade (HG) kits usually come in 1/144 scale though some are manufactured in 1/100. High Grade kits are moulded in multi-colour plastics so the kits does not require building if so desired. While often considered a good starting or beginner line for young or inexperienced gunpla builders, they are usually more limited in movement than the higher grades.
Master Grade (MG) come in 1/100 scale and are the next step up. These kits are more advanced than HG kits with better range of movement and some later kits have an internal skeleton like frame. RG kits are moulded in variable colours with but again benefit from painting.
Real Grade (RG) come in 1/144 scale and are a more recent line. Real Grade kits are somewhere mix HG and MG lines. They have the smaller and are less expensive like High Grade kits but have an inner skeleton like later MG kits plus a higher parts count the rivals MG approaching perfect grade quality. Due to the smaller size of RG kits extra care is required during parts clean up make sure all parts kept movable to design. While moulded in colour they will benefit from painting for better presentation.
Perfect Grade (PG) come in 1/60 scale. Perfect grade kits are high level and very detailed kits containing extreme detail many moving parts and some even have inbuilt LED lighting the downfall? All this comes with a price to match often with kits usually in the one hundred to three hundred dollar price bracket, nevertheless many Gundam builders aspire to this level.
Listed above are the five basic versions of Gunpla, however if we include both SD and Ver. Ka there are seven different grades.
Super Deformed (SD) could be classified under No Scale but do have their own classification of SD on their boxes. These kits are scale down child like or "chibi" versions of normal gunpla kits. They have lower part counts and are a good way to for to get younger builders started in the hobby or for a fun couple of hours for a more experienced builder.
Ver. Ka are a line from Bandai with concept designed by Hajime Katoki. These kits based in some of Bandai's normal Gunpla but upgraded with tweaked design and parts hence Ver. Ka short for Version Katoki.
A possible eighth line also exists, this being the G'guy line from the Gundam Build universe. G'guys are Gundam that resemble large teddy bears. Technically these mostly come under HG titles with some under the No Grade level. All subjects are 1/144 in scale but some are of such simple construction they can be put together in a matter of an hour or two by a child with very limited model or Gunpla experience. These are a great way to introduce young children to the hobby