Basically, what everyone else said.
Boron only has 3 valence electrons (ns2 np1). So, with three flourine's attached, you'll form a covalent compound by sharing electrons. The compound now has 6 electrons, and it needs to have 8 (octet rule, though there are exceptions for some atoms). Anyhow, it can accept two more electrons to fill its octet.
Lewis Acid = Donate H, Accept e-
Lewis Base = Accept H, Donate e-
So, yes, BF3 is a Lewis Acid. I think when a Lewis Base donates two electrons to the compound, it is called a "coordinate covalent bond." If the base is removed from the acid, it will take its two electrons with it.
There you go.
Oh yeah. Ionic compounds have MAJOR electronegativity differences between the atoms forming a bond. Since one atomic nuclei attracts electrons far greater than the other nucleus, it will cause cation (loss of an e-) and anion (gain an e-) formation. Then, you'll form some molecule held together by ionic bonds in a crystal lattice.