When people talk of a stall converter they mean an aftermarket torque converter with a higher than stock stall speed.
Stall speed is (in over simplified terms) when the converter essentially locks.
When you put a larger cam in an automatic car, you need a higher stall converter so that the cam can get into is power band before the converter hooks up, or the car will bog off the line and might not idle well (depending on how bad the mismatch is)
Therefore, a stall converter is a torque converter, but a stock converter would not be appropriately called a stall converter.
Technically, converters don't lock, unless they are lock up converters, which are rarely used in performance applications. That involves using a secondary friction application like a clutch band.
Think of it this way. There is no mechanical direct drive on cars with automatic transmissions. If there was, your car would die at a stop. Torque converters are a fluid drive coupling that motivates the car to move using hydraulic flow. If you split a conveter vertically down the center you would see two sets of vanes that kinda look like propellers. The front sets drives fluid into the rear set, which moves the car.
Since they are not directly connected mechanically, they will slip at idle, which allows you to stop at an intersection without stalling. Manipulating the design of these vanes effects flow of hydraulic fluid, which effects what rpm is required to move the car forward. Stock converters start moving the car at a very low rpm, which is appropriate for most applications.
With performance use, however, the power band of the motor starts well above idle speed, say at 2500 rpm. The high stall converters are engineered to start moving the car at a higher rpm. This puts the motor in the preformance band of the engine. The trade off is reduced fuel economy and higher heat generation, as there is more slip before launch. For performance applications, it is worth it, even necessary, depending on your engine. For stock applcations, it is horribly innefficient. Lock up converters with a clutch help remove this innefficiency at cruising speeds, but none are really made for high horsepower applications.
Technically, converters don't lock, unless they are lock up converters, which are rarely used in performance applications. That involves using a secondary friction application like a clutch band.
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In reality, there is usually something on the order of 3% slip at full drive depending on design.
In addition, stall speed is not a solid number, it depends on car weight, gearing, engine torque (at stall speed) the phase of the moon, and the month in the Chinese calendar... (I know Shut Up). Any way, a "2500" stall converter might stall at 2700 behind one engine and 2400 behind another, depending mainly (but not entirely) on engine torque around the stall speed.
Also, generally higher stall converters are smaller in diameter, so for instance a 10" converter is usually something in the mid 3000 range whereas a 11" is generally mid to high 2000's, so when someone says "Ten Inch Converter" they mean "Higher Stall"
One way that the basics of converters is frequently (again in over simplified terms) explained is that if you face two fans at each other and turn one on, it will drive the other one. This does not consider the stator of course.
From what I understand (and this was the best explaination Ive heard so far), a torque converter acts like a torque multiplier. On manual transmissions, you have to rev the rpms in order to take off (like 3500 rpm on Boss 302's, for example). A stall converter actually does that job for you by multiplying your engine torque by 2x or 3x to give you enough power to take off. On higher performance engines, your peak torque should be at a higher rpm, therefore you want a stall that will bring you as close to that rpm as possible. When your stall converter "locks up", or when its torque conversion is almost to a 1:1 ratio, you want to be at your peak engine torque so your engine can do the rest.
On a stock "stall" converter, it gets to a 1:1 ratio around 17-1800 rpm, which is well below peak torque on higher performance engines. So if your torque peaks out at 3000 rpm, you are playing "catch up" from 1800-3000 rpm which results in a loss of potential performance. Now, if you had a 3000 rpm converter, you would gain almost 3x as much low end torque at lower rpms that would not finish getting to its 1:1 ratio until you were well into your powerband.
Now, depending on your load or how heavy your foot is into the gas, your 3000 rpm converter could still lock up around 2000 rpm or it could go even higher than the 3000 rpm. (which is why you see manufacturers give a "guesstimation" about what it will lock up at.) On my 351w, I hit my peak torque at 3000 rpm, so I went a little low and got a 2600 stall converter. It may lock at 2200 rpm, or up to 3000 rpm depending on what i am doing, but it works great for me. Hope this helps.