1) A DAC's job is to create an analog sound signal from digital samples. Digital audio is, at it's most basic, a series of digital samples that indicate the amount of pressure change happening at that moment (as determined by a mic creating an electrical signal based on those changes in pressure and then that electrical signal being recorded by an ADC, which is the reverse of a DAC). The DAC takes the series of samples and produces the analog signal that corresponds to the given samples, which in turn allows for the analog signal to drive a speaker so that we can hear the sound recreated.
2) There is a DAC in a SoundBlaster card, just like there is in your built in sound card, but this isn't what you are looking for when trying to do music production. Consumer sound cards generally provide relatively poor isolation from the impact of the computer itself. They also are designed for use with consumer components such as computer speakers that use different output levels than professional production gear and are designed for listening, not production. The DACs that you want for music production are designed to provide strong isolation, designed to provide accurate, analytical sound and provide signals that are directly compatible with professional audio gear such as studio monitors, PA systems and external processing gear. It provides proper ground isolation and other important features that are relevant to working as part of an overall signal workflow rather than just directly feeding speakers.
3)The exact features you should look for vary based on what you want to do, but I'd recommend looking for something with balanced inputs and outputs via XLR. On a lot of prosumer stuff, you can get hybrid connectors that will work with both 1/4" cables and XLR so that you can work your way up as you invest more. You also want to look at the bit-depth and sample frequency. 24-bit 96khz audio is a good starting point for producing music so that it can be mixed down to 16 bit, 48khz with minimal aliasing artifacts and other issues.