It used to be that pc's didn't have sound built in. They had to make expansion cards so that sound could be accommodated; that's why soundblaster became a big hit in the early 90's until ultimately, sound was integrated into every motherboard. You can probably thank Steve Jobs and Apple for that.
Note that you CAN record with onboard sound, but it usually isn't the most efficient way to do so. For one thing, pc sound is usually designed for efficient playback with recording added as a bonus. Low latency recording is not high on the list of priorities when the developers make drivers for onboard sound; this is why we need a dedicated audio interface. Often, external and prosumer soundcards (such as the fast track) will have better, low latency drivers as well as multiple I/O and maybe provisions for instrument inputs which put conditions such as impedance mismatching out of the question.
You can have onboard sound and the fast track both working simultaneously. Used to be that I would route my onboard sound into my recording card so I can hear my games or movies after a hard day's work - I had up to 3 different sound sources at that time. Onboard sound goes to my pc speakers while the 'pro audio' goes to my headphones or monitors. Let's just say I could hear the difference between the 2 and I couldn't trust onboard sound for critical listening. Now, my 1820m does everything audio related - a much simpler and efficient setup.
The simplest way to connect a guitar to a pc is to get one of those usb guitar cables or interfaces... the fast tracks have guitar inputs. You may have problems with monitoring when you use the simple interfaces, though... no speaker outputs in the most inexpensive usb cables and interfaces.
