A few things I’ve learned about batteries lately:

• Rechargeable AA’s are 1.2V, whereas normal alkaline AA’s are 1.5V. I didn’t believe this at first, but it’s usually printed right on the battery (in unreasonably small print). Where this generally matters is things that take many batteries (in my radio that takes 4, it’s the difference between 4.8V and 6V, for example), although most things will work just fine. (The rechargeables often have much higher capacities, though, so it works out… Unless you get something that’s very nitpicky about voltage.)
• Almost all “normal” alkaline batteries: AA’s, AAA’s, C’s, and D’s, are 1.5V. The typical capacity of a AA is somewhere around 1,000-2,000 mAh, but did you know D-cells are often around 15,000 mAh? (Which is 15 Amps if I’m not mistaken, which means it’s got about 25WH of juice.)
• As a consequence of the above, as far as voltage is concerned, you can use a AAA where a D is called for, or a D where a AA is called for, and they’re all the same voltage. It’s just that, as the batteries get bigger, they last a lot longer. (And good luck sticking four D-cells into your camera so it last longer…)
• “Digital” devices will stop working below a certain voltage, which is usually before the battery is fully drained. Unlike a flashlight, which will just get dimmer and dimmer as the battery drains, electronics (think of cameras, for example) will continue working until there’s insufficient voltage, at which point they shut down. Thus the “dead” batteries from a camera (etc.) may continue to work in other things, like a remote control or a flashlight, though the flashlight would, of course, be dimmer than usual, since the batteries you put in would be low.
• It’s possible to recharge alkaline batteries if they’re not completely flat. But don’t try this at home (unless you have the aforementioned charger or a desire to have boiling battery acid in your eyes): alkaline batteries were never meant to be recharged, so ordinary battery chargers will cause the batteries to overheat, ooze acid, or just flat-out blow up. But if you get a charger specially designed to recharge non-rechargeable batteries, it can be done!
• There’s a AAAA battery, and it’s exactly what you think it is. The AAA is a smaller version of the AA, and consequentially doesn’t hold as much of a charge; the AAAA, then, is a smaller AAA which holds less of a charge. You probably haven’t seen many AAAAs, but that doesn’t mean they’re rare. That’s because…
• 9V batteries are just 6 AAAAs in series. (That said, cut batteries open at your own risk!)
• Your 12V car battery should actually be around 12.6V; a true 12V indicates that it’s largely drained. (Your car’s alternator should recharge the battery by providing ~13.8V when the car is running.)
• There’s a lot of information out there about “memory effects” and such, and thus all sorts of confusing, contradictory information about recharging batteries. NiCd (Nickel-Cadmium) batteries suffered from a strong memory effect: if you routinely recharged them before they were completely drained, you would drastically decrease the charge the battery could hold. NiMH (Nickel metal hydride) batteries reduced this effect, and LiIons (Lithium Ion) eliminate it. Thus “topping off” most newer batteries isn’t in and of itself a bad thing. However…
• Batteries still have a limited “charge cycle,” the number of times you can recharge them. Thus recharging your battery any time it dips below 95% charge is going to wear it out prematurely. Where this really matters is laptop batteries: you charge your battery fully, unplug and shut down to bring your laptop to a meeting, and then plug in there. This is murder on the battery. Some laptop battery systems are “smart” about this and will simply not bother charging a basically-full battery, but as a general rule, if you’re discharging a battery, don’t recharge it until it starts to get low…

Edit: An alkaline battery, being, well, alkaline, won’t actually leak acid, but potassium hydroxide.