Using a plectrum or pick to play is one of the most commons way to approach the guitar. The pick is held in the right hand between two fingers, and its tip used to attack the strings on the side of the soundhole. This technique, of course, renders at least two of the right hand fingers unable of playing independently -thus hindering the possibility of complex polyphonic playing. In return, it yields immense benefits in tone control, volume, and speed.
Alternate picking is by far the most used technique in plectrum playing. It involves using a continuous upwards-downwards motion in the picking hand, sometimes even when there are no notes to be played (in order to consistently play upbeats and downbeats with upwards and downwards motions respectively). Tremolo effects become particularly easy to achieve through the use of alternate picking.
Pure downpicking, while useful in certain situations and for certain sounds, is a demanding technique that can quickly become exhausting and difficult to control. It is also rendered impossible by high speed runs, so alternate picking becomes essential for plectrum playing.
Naturally, not every situation can be resolved by employing the alternate picking technique, and sometimes fingerstyle playing, legato playing, sweep picking, or hybrid tapping techniques are a better choice, though alternate picking remains a basic technique, and it should be approached at the beginning of every guitarist's career and perfected on a daily basis.