The term telemarketing usually designates a process of prospecting or selling either over the phone or video conferencing. Besides selling, telemarketing can also include surveying and other techniques of information gathering. One of the recent trends in telemarketing is robocalling - using pre-recorded messages and auto dialers to pitch to a large number of people automatically.
There are several similar terms to telemarketing, such as cold calling or inside sales. Sometimes they can be used interchangeably, however there are subtle differences:
Cold calling refers to a technique of contacting potential customers over the phone in order to convince them to buy something. What makes cold calling different is that this activity strictly refers to contacting new customers who have neither showed interest nor bought anything from your company yet.
Inside sales is a process of remote sales, when sales reps solicit customers online or over the phone. This selling activity requires building a relationship with a potential client and leading them through the whole sales funnel.