Historically, acids and bases are defined as proton donors or acceptors respectively. Another definition for an acid and a base is that of an electron acceptor and an electron donor, and when these interact a dative bond is formed.
An acid is an electron acceptor, and this normally includes a central atom such as Al and Be, two atoms that when they bond covalently they do not end up with 8 electrons in the outer shell. Some examples of these molecules are:
A base is an electron donor and this requires lone pairs, such as those found in covalently bonded Nitrogen. Examples of electron donors are: