USB charger/power tech question

I recently acquired a smart phone, but I suppose this question applies to USB power and specifically chargers of all types.

Can anyone tell me, or point me to a good guide that breaks down information about general compatibility for charging devices that use USB ports for this purpose?

Specifically I have a number of chargers that came with devices, and most seem to put out 5v but a varying amount of mA ratings. So in general I'm wondering what can be used to charge what safely. I believe my phone came with a 850mA charger (at 5v), so would it be save to use one that is 1000mA? What about 500mA? Also my blutooth headset came with a charger as well, but I believe it was significantly lower like 80mA or some such. Additionally the phone came with a car charger, and i'm not even sure what the mA rating on it is, but it has a 2nd port for more devices to charge at once.

So what happens if I plug my blutooth headset device into one of these larger chargers, or the car multi charger? Does it just take what it needs and ignore the rest? or will the higher mA cook it? I looked online for quite awhile and I couldn't find very much useful information on the topic, only anecdotal comments that I didn't find to be of much use.

Also i've noticed that often many of these products don't even seem to specify their mA ratings on shopping sites when you are looking for car chargers ect, and it doesn't help that my headset (Jabra VBT3050) doesn't seem to have an existing manual anywhere.

Anyhow just looking for some clarity on the topic, so I can make more informed decisions about what chargers to use to charge what devices, without damaging any of the devices. Thanks in advance for any information.

EDIT/UPDATE: I found the charger for my Jabra headset to be 5v at 180mA (just adding the detail not that it changes my question really)