When it comes to rented or purchased lists, you may come across vendors or marketers who say, "This email list is totally opt-in!" This means the people on the list opted in to an email communication from someone at some point in time -- the list provider, for example -- by filling out a form or checking a box to receive more content from that provider.
Another type of electronic mailing list is a discussion list, in which any subscriber may post. On a discussion list, a subscriber uses the mailing list to send messages to all the other subscribers, who may answer in similar fashion. Thus, actual discussion and information exchanges can happen. Mailing lists of this type are usually topic-oriented (for example, politics, scientific discussion, health problems, joke contests), and the topic may range from extremely narrow to "whatever you think could interest us". In this they are similar to Usenet newsgroups, another form of discussion group that may have an aversion to off-topic messages.
A tracking pixel is a snippet of code that allows you to track your subscribers’ activity across various channels like your website, landing pages, and newsletters. Then, you also have the option to send emails triggered by certain behaviors. So for example, if a subscriber goes to your site and checks out your product page, a tracking pixel can send an email with a special offer for the specific product the viewed.