It might sound like a simple question but telemarketing means different things to different people. The simple reason for this is that the term "telemarketing" has come to encompass a whole range of different activities. Most outbound calling is telemarketing but not all telemarketing is outbound. Some companies use inbound telemarketing calls where customers are driven to call a number where the call centre agent will then attempt to sell services to them.
Outbound telemarketing is basically all aspects of proactively contacting an existing customer or a prospect to either sell to them on the call or to use the call as an initial step in making a sale later on. It therefore includes things such as welcome calls to new customers, survey or market research calls through to telesales calls where the ultimate aim is to get the consumer or business to buy your product or service.
Telemarketing can be to consumers or to businesses. It may involve a whole range of things such as making appointments, confirming demographic information through to generating leads where a further telemarketing call may close the sale.
The telemarketing industry is huge and the UK is a heavy user although its use has been restricted by The Telephone Preference Service. This site contains huge numbers of articles about telemarketing and if you are still at the stage of asking "what telemarketing is", then I suggest you take some time to familiarise yourself with these articles.