Super-Deformed or SD GundamSD Gunplas are small plastic kits which are not presented in a proportioned scale. In anime, it can be resembled to as Chibi version – with overly large head. These are easy to build and great for beginners but offer limited articulation or poses and also require paint and detailing to make the kit looked really finished. The most famous line of SD is the BB Senshi (SD Gundam BB Warriors).
High Grade – 1:144 ScaleHG or high grade kits are approximately 13 cm tall and introduced in 1990. It is proportioned to the original Gundam and scaled 144 times smaller than the real life-size Gundam (as shown in anime, movies, manga or videogames). These kits displayed a better range of articulation and greater poseability.
Master Grade – 1:100 ScaleMG or master grade kits (approximately 16–19 cm tall) are introduced in 1995 and scaled 100 times smaller than the real life-size Gundam. These are manufactured using higher quality plastic resulting higher standard molded parts. Since this is bigger than SD and HG, it is more expensive and complicated to build. MG models offer superior and wider range of articulation and poseability. Some of these kits include specially designed stands for added display ability. MG kits are very presentable even if it’s just assembled straight from the box. Adding paint, markings and other details can greatly enhance its appearance.
Perfect Grade – 1:60 ScalePG or perfect grade kits (approximately 30 cm tall) are introduced in 1998 and are proportioned 60 times smaller than the original life-size Gundam (as shown in anime, movies, manga or videogames). These kits are made of even higher quality plastic and metal components. These are much more complicated to build and include parts that have to be secured with screws. These are really expensive and require days to complete the construction.
High Grade Universal Century – 1:144 ScaleHG Universal Century series or commonly called HGUC was released in May 1999. These are 1:144 scale and gave chances to release versions of many mobile suits for the first time in kit form using refined designs and modern injection moulding. New designs with new mechanical improvements. Starting in 2010 the HGUC line incorporated mobiles from the alternate timelines that had not received a HG. In 2013, Bandai launched the “All Gundam Project” in conjunction with the Gundam Build Fighters to release all lead Gundams with the universal joint technology in HG line.
Real Grade – 1:144 ScaleRG or real grade kits are introduced as part of the “Gundam 30th Anniversary.” It is scaled as HG kits but take design elements from MG and PG line such as an inner skeletal frame and achieve the largest range of motion in each joint.
Most information are taken in this source.