Differences in DAC/Amp

I'm no expert, but from what I know the differences are that the DAC only serves as it is named, a Digital to Analog Converter, in form of a USB connection; an amp just amplifies sound.

Some DACs come with some sort of amplification, though it's usually very little.

You can find a lot of Amp/Dac combos though, a device that both amplifies sound and feeds it to a usb connection.

..It amplifies

whether it amplifies volume or amplifies detail/quality.

From my understanding .... 

A DAC is a Digital to Analog Converter.  Most audio (music) is stored electronically now - in digital form (MP3, FLAC, WAV) whether it be on CD or other medium.  In order to hear the sound, the digital stream must be converted to an analogue signal.  This is done by means of a DAC.  You'll find that all portable players and PC sound cards will have their own inbuilt DAC, otherwise you'd never hear anything.  Buying a stand alone DAC is so that you can buy pass the (lower class) existing DAC for your onboard sound etc, and take advantage of the (hopefully better) DAC in the dedicated DAC device.

An AMP's main role is to amplify power - increased loudness is a by product of amplification - but it has other affects as well.  Rather than me explaining it - here's a wikipedia link that covers some of the basics.  Hopefully one of the seniors here can add further to the explanation.  It's a topic that interests me also :)