Bronsted defined an acid as a substance, which has a tendency to give a proton $\left( H ^{+}\right)$, on the other hand Lewis proposed an acid as a substance, which is capable of accepting a pair of electrons. As $H _{3} O ^{+}$, $NH _{3}$ and HCl has $H ^{+}$ ions, these are Bronsted acids whereas $BF _{3}$ is an electron deficiet molecule, thus be have like a Lewis acid, i.e., electron pair acceptor.