Tim realized that when he brought a positively charged piece of cloth in contact with a negatively charged sock that they both stuck together. Why did that happen?

This was due to static electricity or electrostatic induction.

Explanation:

Electrostatic induction, is a redistribution of charges in an object caused by the electric field of a nearby charge. For example, if a positively charged piece of cloth is brought near negatively charged sock, the mobile negatively-charged electrons in the sock will be attracted by the external charge, and move to the side of the metal facing it, creating a negative charge on the surface. When the electrons move out of an area they leave a positive charge due to the metal atoms' nuclei, so the side of the metal object facing away from the charge acquires a positive charge. These induced charges disappear when the external charge is removed.

Static electricity is usually caused when certain materials are rubbed against each other, like wool on plastic or the soles of shoes on carpet. The process causes electrons to be pulled from the surface of one material and relocated on the surface of the other material.