Since I've been building quite a lot of gunpla maybe I can help you out answer this question. Let me go through each grade that I know of.
Advance Grade (AG)
This grade is actually the simplest 1/144 Bandai has to offer. It's a kit well fit for young ones due to the easiness of the build and it's limited movement (only head and arms move). You don't actually need tools to build this. This is definitely not a good kit to start with if you want to know the basics of gunpla.
High Grade (HG)
This is considered as the best gunpla starter kit for newbies. It is simple to build, has nice few details and sufficient articulation. The building process you'll find here would be quite similar to the MG and PG though it's less complex. Thus if you master building this, you would have less trouble building the bigger ones. All recent HGs have a size of 1/144 (there were 1/100 before). Most people collect HGs because they almost have the complete set of mobile suits (MS). A lot of modelers also choose to buy these kits since first they are cheap and second you have lot's of room for creativity for this one. With the addition of Gundam builder weapon sets which was introduced a few years ago, the HG truly became much more desirable since you can now add a variety of weapons to almost any HG kit. The downside of HGs are that articulation and gimmicks are limited plus colors are less accurate. Overall, I love buying this grade since, it takes less time to build, fun to customize and experiment with and takes up less cabinet space.
Real Grade (RG)
RG is simply a much detailed version of HG. It offers more feature, accurate design and colors and better articulation than the HG one. It has an inner frame which HGs don't have and more stickers.The only problem one would encounter with RGs are the loose parts plus those foil stickers that comes off easily (which is why most resort to painting parts rather than using stickers). Expect that this grade is more expensive than HGs (and can sometimes cost as high as an MG).
Master Grade (MG)
In my opinion this is the best grade out there. Why? Well it is very detailed, highly articulated, has lots of gimmicks and affordable. It has everything your looking for in a gunpla. It comes at a size of 1/100 plus the features it has it a lot more better than it's HG version (actually to give you an idea, an MG is basically the bigger counterpart of an RG). Their structure is usually accurate to what you see in manga/anime and it can only get better. Unlike before, most MG's now come with LED systems which makes your gunpla more realistic. They also have decals aside from stickers (water slide/ dry decals). The only reason why some don't dig in to this grade is because their tedious to make and can sometimes be priced high (usually the Ver Ka. sets).
No Grade (NG)
They usually come in various scales with less accurate designs. Though no grades is not popular, people still buy them since they offer MS designs not found in other grades.
Reborn Grade (RE)
This grade is just new and is basically a hybrid of the HG and MG. It has less details and articulation but comes at a size of 1/100. I have yet to build one but I think that REs are similar to No Grades but are only much better.
Perfect Grade (PG)
Well as the grade mentioned, it's perfect. With a size of 1/60, it offers various features and the best articulation and design for the MS. However they are very expensive. It's totally not a good gunpla for newbies unless you really want this grade. Building it can be quite complex but the final product would surely make it worth the hardship. I don't collect this stuff since not only does it consume a lot of cash it also takes up lots of space.
To sum up, if your a newbie what should you buy?
Well I recommend going for HGs but if you want a bigger mech go with MGs. Those two are the best grades Bandai has to offer. RGs and REs comes in second and third respectively. Well that's just my opinion but the final say will still be yours.
What about SD's (Super Deformed)
Hmmm... they are the cartoonish version of the MSs. They are made cute to compliment the small size. They are the most difficult gunpla to paint due to the details being limited and small. For me, they are gunpla's suited for young ones unless you're one of the few who challenged themselves to paint and customize these tiny mechs.
One last thing, check out dalong.net if you want to know more about the model you'll purchase. It's a great site that features most of the Bandai product line.