I'll try my best to break it down for you simply, and you can determine what is the most important for you. The holsters I will refer to are all leather, though kydex holsters of similar construction will have many of the same qualities as far as concealment is concerned.
OWB, or Belt holsters:
You will see two major types of these, with all manor of different variations of each. Keep in mind that belt holsters will probably be more comfortable, but also more difficult to conceal. You must make sure that your cover garment is long enough to cover the bottom of the holster even if you have to reach up above your head at the grocery store, etc...
The first type I'll mention is a variation of the Askins Avenger, or Bruce Nelson #1 holster. This holster is typically made from a single piece of leather bent over the top of the slide, having a tunnel belt loop behind it, and typically another belt slot to the rear of the pistol. This holster is basically the easiest to get a full grip on the pistol with, and the easiest to re-holster since the reinforced mouth won't collapse. If the holster is made correctly it is balanced very well so it is also very comfortable for long term carry. The draw back to this style is that it sets farther out than many other holsters so it is more difficult to conceal.
The next holster to consider is the pancake style holster, or the belt slide. It is usually constructed of two pieces of leather sewn together with the pistol sandwiched in between. The placement of belt slots on either side of the pistol draw this holster tightly into your side so it is very snug, and much less likely to profile, or"print", than the Avenger style holster. The drawbacks to this style are that they are slightly more difficult to get a full grip on the gun while it's holstered, along with the increased possibility that the mouth of the holster may collapse while the pistol is drawn, making re-holstering more difficult. Keep in mind that while the gun is held tighter to your body, facilitating the use of lighter weight cover garments, it is still a belt holster and therefore has the normal strengths and weaknesses.
There are other variations of holsters that allow easy on and off removal of the holsters without removing your belt. They are the paddle holster, and the snap holster. Paddle holsters tend to share the characteristics of the avenger style, while the snap holsters tend to be more like the pancake type of holsters. I include them because you're likely to run across them in your search.
IWB, or Inside the Waistband Holsters:
This type of holster can also be categorized in two major styles. The first is the standard IWB holster, and the second is differentiated by the ability to tuck your shirt in over the pistol and the holster. Rather than trying to discus the structural differences, I will list the benefits and draw backs of each as I see them and leave the rest up to you (as there are so many kinds of holsters and body types that this post would end up becoming a book if I tried to cover all the possibilities).
The benefits of the IWB holsters are the ability to use a shorter, lighter cover garment. Even an open button up shirt will suffice as long as the pistol is placed on the body appropriately. If the cover garment raises as you lift your arms, no one will see the holster since it is inside your pants. These holsters are similar to many pancake style holsters in their drawing and re-holstering characteristics. The main drawback is that some people find having a holster inside their pants is completely uncomfortable, mentally, physically, or both.
The big difference in the tuckable holsters is the ability to tuck your shirt in and completely hide the holster without looking out of place if you are required to dress in a specific manner. A good tuckable holster embodies the saying "Out of sight, out of mind". The extreme concealment does come at a price. Getting your pistol into play when you need it takes longer, and is much more obvious than the other styles of concealed carry.
Here is another way to look at it, but this chart is largely based on my personal opinions so YMMV
Holster............... Presentation.......... Comfort.......... Concealability
Avenger................ 4........................... 4..................... 1
Pancake................ 3........................... 3..................... 2
IWB...................... 3........................... 2..................... 3
Tuckable............... 1........................... 2..................... 4
Hope that helps. I'm sure your gonna get all kinds of answers so maybe this can put things into perspective a bit.
