Escrow is simple the process by which Title is transferred from you to the buyer. In most states this is performed by an escrow or title company and in some areas attorneys assist.
If you are obtaining an offer from an Agent he/she will recommend which local company they prefer. Most escrow companies are consistant with their fees and who you or your agent chooses, generally comes down to relationships with certain title officers your agent may have.
In the event your transaction is with a private party, below is a list of companies that are quite capable of handling the escrow process.