This is a type of reaction mechanism that is seen in case of nucleophillic substitution reactions.
It is BIMOLECULAR NUCLEOPHILLIC SUBSTITUTION mechanism.
In this kind of mechanism two molecules decide the rate of reaction, so it is BIMOLECULAR mechanism.
The whole mechanism proceeds through a single step. That concerted step involves the bond breaking and bond formation simultaneously.
It involves the Inversion of symmetry .
The backside attack of nucleophile takes place as shown below.
The Unconditional Guru 