Transients
Transient is the term used to describe the initial part of the sound created when an instrument starts sounding a note. It often contains non-periodic oscillations (noise rather than resonant pitches) and higher frequencies than the harmonic content of the ensuing note.
For guitar, depending on one's playing style, guitar setup, and means of sounding the strings (plectrum vs. fingers)), there can be very noticeable transients on the beginning of notes. The transients will be more apparent when playing loudly, using a pick, and using a setup with low action. A slapping technique will also prominently feature transients. On the other hand, playing softly using finger picking will reduce the relative presence of transients compared to the sound of the fundamental and harmonic frequencies of the string's resonance.
Transients are neither good nor bad in general, but as a player you should be aware of them and attempt to master using them to increase your expressiveness. For example, you don't want every note you play to have the same level of transients, especially if you are using high levels of transients - this may sound overly percussive and noisy. Rather, you should ease back on your picking strength or use some legato techniques to reduce the transients until you have an accented note, where you can dig in with the pick to get a noticeable transient to help accentuate the note.
Also, there are devices such as the E-bow or Sustainiac as well as volume swell techniques that allow the strings to swell into volume, either containing no transient, or effectively muting that portion of the sound.
You can also use compression to limit the relative volume of transients.