Sharp is the phenomenon in music when we increase a pitch by a halftone or semitone, making it sound higher. The symbol used to indicate sharp notes and chords looks like this: #. Sharp is the opposite of flat, because flat means decreasing a pitch by a halftone, making it sound lower.
Sharp on guitar and ukulele
By “halftone up” on the guitar or ukulele, we mean going to the next fret toward the bridge. In the case of the guitar:
For the ukulele, sharp looks like this:
Sharp on piano
By “halftone up” on piano — going up to the next key. Note, that it can be a black key or white key (in case there is no black key). For instance, when you increase a C by a halftone, you’ll name it C sharp, as shown in the picture below. Please note that C# equals Db. This special situation is called an enharmonic equivalent.
Sharps in sheet music
In sheet music, the sharp(s) can appear as a key signature(s), standing next to the clef. Below, you can see an example of sheet music with two sharps. This means that all F and C notes need to be increased by a halftone, becoming F# and C#.
The sharp can also appear in one bar. Take a look at the picture, in the second bar, the sharp appears next to the G note, turning it into a G#. Please note (!) that this sharp refers within this bar to all notes G of all octaves and it works only within the bar where it appeared.
Regarding the picture above, it’s also important to mention that sharps can be a part of a chord name, showing us that the entire chord is played a halftone higher. In the picture, you see the F#m7 chord in the second bar as an example.