A guitarist plays notes either by playing one of the six open strings on the guitar, or by pressing down one of the six strings at a certain point (fret) on the neck of the guitar. A pianist

presses (strikes) the piano’s keys in different places on the keyboard to create notes. Both musicians can play a wide variety of notes including all naturals, sharps, and flats. Thinking of the

difference in octaves, a piano has 88 keys (notes) in over eight octaves, where a typical guitar (and its six strings) is capable of four and-a-half octaves.
The strings of the guitar are tuned to E, A, D, G, B, E, low to high, and have corresponding notes on the piano. Middle C (the note C) on the piano is the same as the 5th string, third fret on the

guitar. Thus the guitarist’s lowest note, E, corresponds to the E below middle C on the piano. Some say notes to be played can be easier seen on the piano, while others claim finding the same notes

on guitar as being the easier task.
You may ask the question, can a guitarist play music written for the piano? Or can a pianist play music written for the guitar? The answer is yes with the following caveats. It can be quite a

challenge for a guitarist to play music written for the piano. Whereas a piano player can play 10 notes simultaneously (ten fingers), a guitarist can play only six notes at the same time (strumming

the six strings of the guitar). However, an experienced guitarist can “arrange” piano music into a recognizable form that can be played on guitar. 
To further define the differences between piano and guitar, it might be helpful to think of the bottom three strings of the guitar (the lowest sounding, or bass strings) as corresponding to what

the left hand plays on the piano, and the top three strings of the guitar (the three thinnest strings) as what the right hand plays. This is a very general statement (and is not proven in fact or

practice) but helps separate and define the two instruments. The root of a chord on guitar is usually played on one of the bottom three strings with the remaining strings used to complete the

chord. And as a general rule, the left hand of the piano is used to play the root of a chord with the remaining fingers (in both the left and right hand) used to complete the chord.
A guitar and piano can easily play in unison, showing that all the strings of the guitar have corresponding strings on the piano. Both are capable of complex and full-sounding music on their own

and also can be played to beautifully compliment each other.
A final difference is that single notes on the piano are created by the hammer in the piano action striking two or three strings (depending on where the notes are played on the keyboard), while the

same note on a guitar is generated by one string only. (Please note these are generalities to view the overall picture of the relationship between the two instruments and not hard-and-fast facts.)