There are two possible answers to this question (so I'll try to answer both!).

If you mean table salt, or the salt that we often buy from the grocery store and add to our food, this is made from the two elements sodium and chlorine. Due to the way that these two elements exist, sodium has an electron that is easily given away, and chlorine wants to take on an extra electron. Chlorine does this naturally by forming a covalent (or sharing) bond with its self, and is found as a gas, that is toxic. Sodium does this by making a solid structure made of all sodium that is highly reactive to water (and will explode/catch on fire). When the two are combined however, they exchange one electron and then have opposite charges, which attract, and can make the solid, non-toxic solid we use to add flavor to our foods.

In general, a salt is made of at least two different types of atoms, one positively charged, and the other negatively charged, such that when they are present together they form an ionic bond. An ionic bond, unlike the covalent bond, takes the electron (non-sharing) and creates two charged atoms, and is in general weaker. In addition, when molecules with this type of bond are added to water, they dissolve easily.