A Guide to Gunpla Scales and Grades

Gunpla “grades" are scale and complexity indicators. You should be able to start building any grade without difficulty, as long as you follow the pictogram instructions. More recent Bandai kits also have English Instructions.

1/144 scale

• High Grade (HG) which is a relatively simple build, often without any frame and mostly lack details. HGs are the cheapest among the grades, most abundant, available, and the HG line has the widest range of kit designs and variants. Most MS designs are released as HGs. Some HGs have rudimentary "pseudo" frames, like the Sazabi and K'shatriya. The HG series are appended by the anime series Universe, ie, HGUC, HG00, HGAC, etc. The HGIBO is the first line that feature internal frames.
• Real Grade (RG) has an internal frame like modern MGs. RG frames are mostly pre-assembled in sections and the armor parts are heavily detailed. Only select MS designs (main protagonist and antagonist suits) are released as RGs.
• Entry Grades (EG) and First Grades (FG) are 1/144 scale kits intended for younger builders for quick building, do not have polycaps, and lack articulation and detail. EGs are molded in 3 colors, while FGs are molded in white PS plastic. The new Entry Grade Gundam RX78-2, however, features better articulation.
  
• 1/200 Speed Grade. These are preformed and prepainted kits that come in a single runner.

1/100 scale

• 1/100 HG is just like the name implies. They are 1/100 scale High Grade kits released in the 90s. The only series released in this scale/grade are the Gundam Wing and Gundam X (afaik).
• 1/100 Non-Grades (NG) or simply 1/100 scale (the NG is user-appended) are similar to HGs. They don't have internal frames, and they often lack details. They are the cheapest among 1/100 scales, and MS kits released in NG/100 are from the non-UC series.
• RE/100 (Reborn series) are "rebranded" 1/100 scale. They are similar to NGs that they do not have internal frames, but do have ample details. The kits released in this line so far are from the UC series.
• 1/100 IBO. These kits come with simple internal frames and detailed armor parts. The articulation is rudimentary, but still good compared to other regular 1/100 scale.
• 1/100 FM. Stands for Full Mechanics and replaces the 1/100 IBO full frame tag.
• The Master Grade (MG) has an internal frame, often with intricate details and complex articulation mechanisms. Most modern MGs (2.0 and 3.0) have full inner frames, whereas older ones only have partial inner frames.
• The Master Grade verKa. A distinct line of Master Grades designed, Engineered and Produced by Master Designer Hajime Katoki. The verKa line is also easily identifiable with its distinct white box.
• Master Grade Extreme (MGEX) - The MGEX is marketed as a next-level Master Grade that goes beyond the detail of its predecessor. First in line is the MGEX Unicorn, which is marketed to have improved articulation, detail and houses an RGB LED lighting system along with a dedicated MS Cage. Next in line is the MGEX Strike Freedom.
  
• GFF-MC and Metal Build. These are composite figures, with metal die-cast and PVC plastic parts. Though categorized as non-scale. these figures are based on 1/100 scale heights. Most are standalone figures, though some have swappable parts, like the MB Exia/R3. GFF-MC Stands for Gundam Fix Figuration Metal Composite. Up until recently, the GFF-MC line have all been UC MS, but the F91 is released as GFF-MC feat Metal Build, while the Wing Zero Custom is released as GFF-MC.
• High Resolution Model (HiRM). This new line of metal composites is a hybrid figure/kit comes with a hybrid die-cast metal/plastic frame, and plastic armor parts still on runners, which gives collectors the chance to assemble the parts themselves. The first to be released in this line is the IBO Barbatos and P-Bandai Barbatos 6th Form. The Wing Zero and Red Frame Astray were each released a year after each other. The most recent release was the HiRM God Gundam, which were preceded by the Wing Endless Waltz and Astray Noir respectively.

1/60 scale and higher

• 1/60 kits are just scaled up HGs/NGs, with no inner frames. These kits lack detail and with minimal articulation. There are only a few kits released in this line (like the Exia, Strike Freedom, Shining/God Gundam, Freedom, et al), in very low circulation.
• The Perfect Grade (PG) line has plenty of parts owing to its size. Earlier PGs have very complicated frame assemblies (up to the GPO1/FB), with modern ones having a <simpler> yet robust frame assembly (starting from the Strike). The latest in the PG line is the Perfect Strike Gundam, which basically the PG Strike with Launcher and Sword Striker packs. 
• The Perfect Grade Unleashed (PGU) - Is an upgraded PG, with a more complex frame and armor assembly as well as metal detail parts. The first in the line is the PGU RX 78-2.
  
• 1/35 Jumbo Grade is really more like a scaled up HG, no internal frame and limited in articulation and detail.
• 1/48 Mega Size. This line is simply scaled-up HGs, with very minimal articulation and detail. These are mostly display pieces.

SD Kits

SD or Super Deformed kits are Gunpla kits that are not based on any scale, but rather on deformed proportion as basis for cuteness, specifically having very large heads and comically anime eyes. They offer a very limited articulation and require much more attention for details and painting. To compensate for lack of detail, some SD kits have very large sticker pads included, while other SD kits have an extra gimmick not shown in their normal counterparts. One of the most common SD kits in Japan are based on the Plamo-Kyoshiro’s Musha Gundam line.

The most current iteration of SD kits is the SD Gundam EX-Standard (SDEX) which are based on the current Gundam models in SD size. They are cheaper to produce and have more articulation than the previous SD kits, though is more lacking in plastic production as some parts (especially in the limbs) are left hollow to compensate on its cheap cost.

Recently released is the SD Cross Silhouette series. These SD kits come with and can be assembled on a generic internal frame and allows builders to mix and match parts from different kits as well as set different proportions by using the Cross Silhouette frame.

Source: https://www.ohmygundam.com.my/blogs/blog/gundam-size-comparison