There are several different ways you can go with kitchen cabinets. In this post, we're not going to focus on the style, but rather the ways in which you can purchase your cabinets. This is the same for both kitchen and bathroom remodel; whether you're looking for a vanity or an entire kitchen set.
If you were to get your cabinets from a dedicated cabinet shop, they are more than likely going to be custom. That is, they are tailor made to the exact layout of you kitchen. An estimator from the company comes out to your abode, takes measurements of every wall and protrusion, gets an idea of what you would like in the cabinets and designs the cabinets to fit perfectly in the space you have. This has the benefit of utilizing every square inch of your kitchen efficiently so you get as much storage or counter space as possible. The cabinets are built to varying widths and shapes in order to maximize their utility. These are, however, usually the more expensive route to take. Sometimes costing double what other methods can, but usually no more than a couple extra thousand, which, is only about 10% more.
Prefabricated cabinets are what you would get from a big box store or a cabinet supplier (not manufacturer). These are generally a little less than custom cabinets, but have the downside of being generic cabinets. You can get them in a number of different styles, but they wouldn't be unique to your home, such as would custom cabinets. Since they only come in set widths and shapes, they may not utilize every square inch of your kitchen efficiently. Either someone from the supplier comes to your house, or they show you how to measure your kitchen and you bring in the drawing. They try and fit their standard cabinets into your kitchen. But let's say your kitchen is 9' 4" wide. You need one 36" sink base for a sink, that leaves you with 6' 4" left. Well, you can put 3- 24" cabinets in there but you're still left with 4". Since they can't make one cabinet bigger, let's say 28", they would just give you what is called a filler strip. It's a strip of wood the same color and style as your cabinets that is 4" wide to fill in the gap either between the cabinets, or between the last cabinet and the wall. This is then wasted space, where with custom cabinets, they can make them to any size to suit the space. The perk of pre-fab is that they are generally a little cheaper than custom cabinets because they are mass manufactured somewhere, usually with lower costs of labor, and shipped to your supplier.
Un-assembled cabinets are the cheapest, generally, of the lot. They usually have the lowest selection of styles, but again, they are usually the cheapest. To get the maximum amount of savings out of these, it would generally be something you, as the homeowner, assemble and put in. If you have to pay someone to assemble them, you lose out on some cost savings. But they can still be the cheapest of the bunch. The reason they are the cheapest is because they come un-assembled in a flat box. They can ship more out for cheaper, and also the labor to assemble them isn't covered by the manufacturer. There are a number of suppliers for these kind of cabinets, but not all are created equal. Make sure you're getting at least plywood boxes, instead of mdf, ball-bearing drawer glides, and concealed hinges. You want to save money, however, you don't want cheap cabinets.