headloss 

There are actually a bunch of great bottom bracket guides floating around on the web... but I'm not sure whether they answer your questions or not. Read the links, then ask questions that the links don't answer for you... then it will all make more sense.

Bottom Bracket Tech Breakdown
Bottom Bracket Standards - Slowtwitch.com
Park Tool Co. » ParkTool Blog » Bottom Bracket Standards
FEATURES: THE BOTTOM BRACKET BREAKDOWN
Complete guide to bottom brackets - BikeRadar

Big picture: There are four primary categories for bottom brackets (I could break down further, but trying to keep concise).
1. Square-taper. Tried and true. Steel spindle that will twist under a powerful rider. Available in loose balls or as a unit that has the bearings built in. Formally, it's called a "cartridge BB" but I prefer to call it a "unit" to avoid confusion. One huge advantage of square taper is that with loose balls, you can set up the chain line however you want by changing the spindle length.

2. Octalink/Isis. Used a larger diameter spindle which resists twisting force. It has a more solid feel under a powerful rider. Problem is, the bearings had to be made smaller in order to fit between the BB shell of the frame and the now larger spindle. Small ball bearings are inherently less durable, which is the downside of Octalink/Isis. Manufacturers attempts to work around this by using variations of roller bearings and needle bearings. The cheapest and least-durable examples used small ball bearings only while the more expensive ones (Octalink, and SKS-ISIS) used complicated combinations if various types of bearings.

3. External/Outboard. (Hollowtech II, MegaExo, GXP, Ultra-Torque, Power-Torque). Kept the larger spindle diameter (24mm) of Octalink/Isis but moved the bearings into cups that sit outside of the frame, allowing use of larger ball bearings that are more durable. The negative of moving to an external bearing is that they aren't as well sealed, in general, as the cartridge-unit bearings above. The bearings used inside these external cups are typically called "sealed bearings" but that is a sort of misnomer as they are not necessarily sealed... these are better called "cartridge bearings" by industry but that can be confusing in the bicycle world, where "cartridge bearing" used to mean the bearing-unit used for many square-taper BBs and all Octalink/Isis BBs. It's a problem of having one word mean two entirely different things. These are what I prefer, in that they are threaded and universal and backwards compatible with just about any bike out there (so long as you can find the appropriate cups). The only down side is that the bearings are more exposed, but you can buy better sealed bearings and the cartridge-bearing itself is easily replaceable from a number of bearing manufacturers. I like them because they eliminate some of the proprietary parts... you use this style of bottom bracket and you will still find a replacement bearing fifty years from now... the same can be said for the original loose ball square taper, but what a p.i.t.a. those are.

4. There are a slew of new BB "standards" that have come out after the introduction of externally-threaded bottom brackets. The later styles are press fit into a frame. The bearings are either directly pressed in, or they are placed in a cup that is pressed in. The basic approach is the same for all examples, and broken down in the links that I posted above. At this point, various standards have more to do with improvements in frame construction rather than actual improvements to the BB itself. The actual bearings haven't changed at all since step 3. in many cases, the change is how they are held in the frame. The biggest difference is that a bunch of the "standards" moved to an even larger diameter spindle starting with BB30 (30, as in 30mm spindle).

So, it's really about moving from a 17mm diameter square-taper to a 24mm spindle to (in some cases) a 30mm spindle. That's the major difference between one standard and another.

Those are the links I bookmarked a while ago when I was trying to sort it all out. That will give you an idea of the different styles and why they exist. It still won't tell you why you would pay more for a boutique brand vs. a basic Shimano but you can search forums on the internet to answer those questions. Honestly, for square taper, I don't think there is a good reason to pay a premium on a cheap to intermediate level bike... because frankly, you'd never wear out the bottom bracket on a bike that sees minimal use. If a neighbor or relative needs a new BB, I generally go with whatever is cheap. I would only buy a premium/boutique brand if the cheaper variety didn't last due to high mileage or harsher conditions (frequently riding in the rain/dirt). I've had good results with the cheap Shimano BB's. Even the cheaper/no-name BB's will last if they are kept clean and lubricated.
I think the only time that I'd immediately jump to the premium brand is if I was replacing a BB on an ISIS Crank/BB because the SKF version has a very good reputation (whereas every other ISIS BB that ever existed has a terrible reputation, probably the most despised "standard" out there).