There are many different "grades" of a Gundam models that Bandai has produced over the years. Some series like High Grade have been around for a long time and include a lot of kits, while others are small collections. In this section we'll cover the basics of each grade.
SD or Super Deformed; is characterized by exaggerated proportions and a cutesy appearance. Many models in the SD line tend to rely on stickers for color separation; this makes them a great choice when learning to mask and paint. In general these models tend to be low cost but also have a low part count.
Arguably the High Grade line of kits is the largest and most varied line of models. Although there are HG 1/100 scale kits, HG is typically used to refer to the line of 1/144 scale kits. HG kits usually have good color separation, but can rely on stickers for some colors. The HG line strikes a good balance between low cost, details and fun playable gimmicks.
Real Grade is a line of detailed 1/144 scale kits. Many builders characterize these kits as having PG detail in a HG size. This range of models includes an inner frame called the "Advanced MS Joint". The small parts that are common in these kits can make them difficult for beginners; however, they can also look great out of the box with only basic panel lining and topcoat.
Master Grade is a line of detailed 1/100 scale models. Most MG models include at least a partial inner frame. In general the MG line tends to feature more, gimmicks, and details and parts than their HG counter parts. The variety of MG kits is less expansive than the HG line, and the price point and part count are higher. In general the MG line of kits have better color separation and limited reliance on stickers; making them a great choice for builders that can't or don't want to paint.
Perfect Grade is a line of 1/60 scale model kits. This line is a more limited line than the others with releases normally occurring a few years apart. The size and cost of these models can make them prohibitive for many modelers, but those that do decide to indulge in one of these models will be surprised by their size, detail and gimmicks.
The Hardgraph line of kits is a mix of 1/44 and 1/35 kits. This is a tough line to pin down. The early kits in this line included Mobile suits, small vehicles and extra weapons; but all of the later kits are 1/35 vehicles and mobile suit parts. These models are often highly detailed, include water slide decals, and sometimes require glue to assemble.
The EX series is a line of mostly ships, add-ons or ground crafts ranging in scale from 1/100 to 1/1700. There haven't been any additions to this line in a long while and reprints of these kits seem to be fairly infrequent. The large carrier type ships like the Whitebase, Albion, Arch Angel, Etc are produced in the 1/1700 scale making a finished ship about 5 inches (12.5 cm) long. Unlike most other lines many of these models require glue to assemble. The small scale, low availability and higher prices for these kits make them a poor choice for beginners.
The Megasize models are a line of gigantic 1/48 models. The quality of these models is roughly in line with High Grade models. Unlike most HG models, the Mega Size are huge, and tower over PG kits. Each model includes a set of water slide decals and a mini Hobby Japan magazine featuring basic tips on how to improve the appearance of the kit. These models are typically around $100, making them an attractive option if you want a centerpiece or conversation starter for your collection.
The Reborn-One Hundred is a newer line of 1/100 scale model kits. The detail and quality of these kits puts them somewhere between HG and MG. These models lack an inner frame, but tend to have a lot of surface detail like panel lines. Unofficially the models in this line fall into the category of popular, but not popular or practical enough to be a MG release.
Advanced Grade / First Grade / Speed Grade / 1/220 / Miscellaneous
Often forgotten about these grades exhibit varying quality; in most cases the cost of these models is very low, but so is the over quality and value. In general these models are skippable and should probably only be considered for younger modelers, or modelers looking for some obscure design.
No Grade is a grade in the same way that "off" is a TV channel. No Grade is a fan name used to describe kits that don't fit into the normal grading system. These kits may still follow a scale like 1/220, 1/144, 1/100 or 1/60; but are otherwise their own line. This "line" is very much a mixed bag and the quality of the line varies considerably.