The Guitar is an ancient instrument, and a relative of the lute. Considerable variations in construction exist, and here we'll briefly describe one, being the Classical Guitar. Most guitars are based around a 6 string, 19 fret configuration, and the Classical Guitar is no different.
Traditionally, the Classic Guitar is played with the fingernails plucking or stroking (also known as strumming) strings, but it's not unheard of for a Classical Guitar to be played with a pick, or plectrum, normally held between the thumb and first finger. The Guitar player can use a variety of techniques to produce variations, an example being vibrato, where the player 'rocks' his hand back and forth on a pitched note, not dissimilar in function to producing a vibrato on an Violin, or other member of the orchestral string family. And like the string family, the Guitar can be notated to indicate up and down strokes across a string or strings, with the same symbols as used by the strings, if this is the composer's intent.
The Classical Guitar isn't, in orchestral context, a very powerful instrument, and the composer should keep this in mind when writing accompaniment. This can be alleviated to some degree by writing tremolo, or rapid strumming up and down, for the instrument, but if the composer intends any degree of solo work, using fingered or picked passages for the Classical Guitar, then giving plenty of harmonic space is essential. Composers may call for some degree of amplification to enhance the Classical Guitar's chance of being more clearly heard, and this could be useful, but this may not always be available, or desired. Scoring for Guitar without amplification, and thinking carefully about suitable accompaniment, will certainly do the development of the composers skills no harm at all.
If the Classical Guitar is to be used in an orchestral non solo context, then the composer should consider it as a colour instrument, to be used sparingly, for effect.