An autoresponder in its simplest form is an email address that sends an automatic reply when it receives an email. Think "I'm on holiday until..." messages. However, when people use the term autoresponder, they normally mean a follow up autoresponder. A follow up autoresponder allows the owner to upload a pre-written series of emails and specify what intervals they should be sent at.
This is commonly used to send someone some kind of course or information on autopilot, e.g. a series of how-to emails 2 days apart for 2 weeks.
In addition, people looking for an autoresponder are going to expect a broadcast feature, even though it isn't strictly within the definition of an autoresponder. The broadcast feature allows the owner to send an ad hoc message to some or all of his or her current subscribers. A subscriber is someone who has subscribed to the autoresponder.
Because the autoresponder accumulates subscribers that can be emailed to, the owner of the autoresponder is said to 'own a list' - meaning s/he has access to a list of people. From a marketing standpoint, this is considered the most valuable asset a marketer can have.
Autoresponders can be hosted on your own server or you can use a 3rd party service. The upside of buying a script and installing it on your own server is no monthly fees. The downside is that it requires a lot of technical knowledge to maintain it so that you achieve high deliverability and don't get blocked because of spam complaints. That's why most marketers choose to use one of the big two companies - GetResponse or Aweber, with Aweber being hands down the most popular.
Aweber is the most widely used autoresponder with the best deliverability. They offer most features you could want in an autoresponder including fairly comprehensive tracking statistics that they call analytics. They do have a rather high monthly fee if you have a lot of subscribers but since you should be making a fair income from a big list, you should consider the cost well worth it.
Because Aweber is the best known autoresponder, no internet service provider is going to block them for spam complaints.
Link : Aweber
Price : From $19.95 / month
GetResponse is another popular choice and has similar features to Aweber. Some people consider them to be more user friendly and for certain list sizes, they are likely to be cheaper.
However, you won't go wrong if you choose GetResponse.
Link : GetResponse
Price : From $17.95 / month with discounts for longer term contracts