When selecting a holster, you may notice that the descriptions say the holster has a certain "level", such as "Super-Gunfighter Level-3 Duty Holster".  What the level indicates, is the number of mechanical retention features that must be defeated to withdraw the pistol from the holster.  This number is typically no higher than three.

• Level 0:  While I don't think I've ever seen a holster advertised as "Level 0", some of your Inside the WaistBand (IWB) concealed-carry holsters could probably be considered "Level 0".  Essentially, they have no retention, and if we hung you upside down the gun would just fall out.
• Level 1:  This holster will have a single retention method, and are most common for concealed-carry, and some professional uses (e.g. military).  For example, a standard Fobus holster holds only via friction, so you must pull hard to overcome the friction and get the gun out.  A Blackhawk Serpa CQC holds only with a latch on the trigger guard, so you must press the button to release it, but once the button is pressed the gun can "fall right out" (no additional friction hold).
• Level 2:  These holsters are primarily used by Law Enforcement (LE), who are at a statistically high risk of being shot with their own firearm, and thus want extra safeguards to prevent it from being drawn.  They feature a combination of two retention methods, such as friction plus button release, or a thumb strap over the back of the slide plus a button release.  The Safariland #6280 Level-2 Duty Holster is an example, using a rotating hood across the back of the slide and friction tension.
• Level 3:  These holsters, again primarily for LE, have three separate mechanical features to defeat before getting the weapon out.  They are for the most safety-conscious people (or those with the most anal department policies), but also restrict your own quick-access to the firearm.  The Blackhawk Serpa Level-3 Auto Lock Duty Holster uses friction, a release button, and a fold-away hood to keep the pistol locked down tight.

Which type of holster do you prefer, or carry, and why?  Let us know in the comments below!