A lewis acid is a compond which can take lone pair of electrons...
AlCl3, FeCl3, BF3, TiCl3, ZnCl2, SnCl4 are all Lewis Acids as they all have vacant orbital in which they can accomodate electrons
Yes in Freidel Crafts alkylation we can use FeCl3, but AlCl3 is a better lewis acid in case of Freidel Crafts alkylation.