In string theory and theoretical physics, strings are hypothetical objects that are believed to be the elementary particles of the universe. If they exist, they would not be point-like particles, but rather 1-dimensional "strings" of energy that vibrate in various dimensions. The image to the right illustrates the various possible dimensions that a string could vibrate in. (Currently, physicists accept the fact that there are at least 11 dimensions in our universe: 1 time dimension and 10 spacial dimensions.) The length of the strings would be determined by Planck length:
It is still not known whether these strings actually exist. They are pretty much the main topic of string theory.