David drops Some knowledge....

Why do electric guitars sound terrible when plugged directly into a computer for recording with applications like GarageBand or Sonar?
 
One way to look at it is this: The output signal of an electric guitar is “analog” and to get good, quality, sound into a computer, that signal needs to be converted to “digital” instead.  In order to change the guitar’s output from analog to digital it’s necessary to use a device called an “audio interface.” In addition to converting audio to digital, an audio interface offers some additional benefits:

• Impedance matching (guitar output to audio inputs)
• Provides additional inputs and outputs
• Generally, offers much higher quality microphone pre-amps
• Usually provides phantom power for condenser mics
• Often come with DAW software to get you started with recording
• They are inexpensive, starting at $99 - less than most pedals

 
If you’re planning to record your electric guitar, bass or any instrument with an electric output, then an audio interface is definitely the way to go.
Thanks & enjoy!
-David