Theorists have long sought a way to obtain a quantum-mechanical description of gravity, the one force that has resisted integration into the quantum framework. In this, one of string theory's most compelling strengths lies. It has the ability to integrate gravity into its own framework, which also includes quantum mechanics, thus providing physicists with the first such theory in history. The difficulty, as stated in Introduction to String Theory, is the fact that the uncertainty principle allows "virtual" particles to "borrow" energy in wild fluctuations provided that they relinquish it within a certain amount of time. These quantum fluctuations become ever more evident as they are examined on smaller and smaller scales, finally leading to the roiling quantum foam of sub-Planck-length distances. This contrasts sharply with the smooth geometrical surfaces postulated and required by relativity.
The main distinction between strings and point particles is the fact that strings have spatial extent, whereas point particles are literally zero-dimensional. It had previously been supposed by physicists formulating quantum mechanics that the elementary particles were points. However, string theory states that the elementary particles are not points but instead are tiny undulating strings. According to the uncertainty principle, the ability of a particle to "probe" an area depends on its quantum wavelength, or the amount of uncertainty in its position - in other words, a particle's sensitivity becomes "blurred" by quantum jitters. As you will see, this proves to be important in the unification of quantum mechanics and string theory.
A particle's quantum wavelength, or its "jitteriness," is inversely proportional to its momentum, which for the purposes of this discussion can represent its energy. Therefore, if a particle's energy is increased, its quantum wavelength decreases - it can be made clearer and clearer - and it can be used to probe smaller and finer objects. According to this formulation, there is no limit to the energy with which a particle can be imbued, so there is also no limit to the smallness of the objects it can probe. This is what gives rise to the quantum foam on sub-Planckian distance scales.
This is where the differences between strings and point particles become most evident. Strings, which are about the Planck length, cannot probe distances smaller that themselves - meaning they cannot even reach sub-Planckian distances. Furthermore, increasing the energy of a string does not necessarily increase its probing ability by decreasing its quantum wavelength. Up to certain energies, added energy will decrease this blurring, but when energy beyond that required to probe Planck-length distances, energy actually causes strings to grow. This makes it less sensitive a probe of short distances. In fact, the energy attained during the Big Bang could theoretically have made a string grow to macroscopic size. This means that a string actually has two sources of the blurring discussed earlier - quantum jitters and its own size. Since adding energy decreases blurring from the first, but eventually increases it from the second, one cannot probe sub-Planckian distances if strings are the fundamental units of the universe - if the smallest unit of the universal hierarchy cannot probe these distances, then quantum foam cannot ultimately affect anything except in a few special conditions. In fact, if something must be measurable in order to exist, it can be said that this quantum foam does not actually exist. It is simply a mathematical artifact arising out of an imprecisely formulated theory.
The most basic possible point-particle interaction is two particles moving so that their trajectories will intersect, as in this diagram. If the particles were ordinary objects - billiard balls are a common example - they would simply collide and be deflected to different trajectories depending on their relative momenta. However, the point-particle collisions modeled in quantum mechanics are a little more complex.
For simplicity, imagine that the colliding particles are a matter/antimatter pair, for example, an electron and positron. When they collide, they produce energy in the form of, for example, a photon. (This is a virtual photon which must soon relinquish its energy.) The photon travels a short distance, then disintegrates, releasing the energy in the original electron/positron pair to produce a new pair. These particles then travel on deflected trajectories similar to those that would be seen if billiard balls had collided. However, this interaction changes still further if the particles involved are supposed to be strings.
If this interaction is described under string theory, the basics are similar, but there are obviously fundamental differences. The strings composing the electron/positron pair collide, as seen in the other examples, and produce a virtual photon - really just one string vibrating in a different way. Therefore,the strings interact by merging into one high-energy string, which then separates into two strings with the same properties (vibrational patterns) as the original two, but with different directions.
There is one major distinction between point-particle interactions and string interactions - while collisions between points happen at a definite point in space that can be agreed upon by all observers, string collisions do not. This is best illustrated with a worldsheet, essentially equivalent to a time-lapse film showing the entire interaction. In this diagram, the location of the first person (white lines) is at an angle to the interaction itself, so this person sees it as happening later than if he had been in the same horizontal plane as the interaction, like the second person (yellow lines). Therefore, while point particles interact in one definite point (they cannot do otherwise, since they are zero-dimensional), strings add uncertainty to the interaction location - they "blur" it, so to speak. If the particle involved here is supposed to be a graviton instead of a photon, this blurring significantly "spreads out" the force delivered, to the point that the infinities formerly obtained when gravity was calculated on short distance scales are replaced by logical finite answers.