A grace note should perhaps be thought of as an extra bit of flourish by the composer and player. Many call a grace note an “ornament” to be used in the ornamentation of music.
In music notation, a grace note is printed smaller than a regular note, sometimes with a slash through the note stem (more slashes can mean more grace notes from the same stem). Although grace notes are written in a particular way for a particular effect, the way they are played is often left to the discretion of the player (or conductor). Normally, a grace note does not take up a place in the music’s structure. In other words, it neither adds or detracts time from the notes around it and isn’t a part of the total time value of a particular line of music.
In symphonic music, grace notes are used to help express the music’s (and composer’s) intent. To create the effect, a flute player, for example, may very quickly press and release a key to create a grace note. A violinist may make a short, sharp movement of their bow the add the effect of a grace note. Playing a grace note on a trumpet could mean pressing a valve and releasing it almost instantaneously.
Many other genres of music use grace notes but in many different ways. A Blues guitarist, for example, will use “hammer ons,” “pull offs,” and “bends” on the strings of the guitar to add a vocal-like effect to the music. The music resulting from these effects (call them grace notes for lack of a better term) are very expressive and moving. As most Blues music is highly improvised, written music (and the grace notes that might appear in the music) is not common.
A Country guitarist may use grace notes in the same ways as a Blues player. But typically, these added notes are shorter in duration, often creating a “clucking” or “plucking” sound. Country music also makes extensive of the steel guitar, which for intent and purpose, is not a guitar at all, but a series of levers and pedals attached to a set of strings, and played with the musician’s feet and legs to create a certain soulful effect. A steel guitarist also uses a metal bar placed over the instrument’s strings to change the pitch of a note or group of notes, making it quite easy to play grace notes.
As far as the history of the grace note, Chopin as known to have used them quite extensively and they were common in other pieces of music from that time. To be proper, the words, “acciaccatura” and “appoggiatura” can be used to describe the occurrence of one or more graces notes from the same stem.