What is the difference between Sn1 and Sn2 reactions?

Both Sn1 and Sn2 reactions are nucleophilic substitution reactions. Sn1 reactions proceed via 2 steps. An example of this is the substitution of a halogen group such as bromine with an OH- group forming an alcohol. In this case the bromine group would leave the molecule first then the OH- group would attack. This is like waiting for someone to get up of a chair before sitting down on that chair. An Sn2 reaction proceeds via 1 step. The OH- group attacks the molecule whilst the halogen is still attached and causes the halogen to he pushed off the molecule. Again this is like pushing someone off a chair whlist you sit down.