Types of "shows"
On most cam sites, there are 3-4 basic types of show/chat sessions available. They are named differently on each site, but the basis is usually similar to the types listed below.
In Free or Public chat, you normally will be visible by anyone who comes to the site, whether they have an account or are just visiting as a guest. It is considered "free" in the sense that there is no pay-per-minute aspect to it, though you may be able to receive tips from members. Public chat is where you appeal to your potential paying members to either tip you, take you private, or join a group or other form of paid show. Some sites do not allow you to show any nudity in Public chat, so make sure to read the rules of each site to determine what you're allowed to do in Public.
Private with voyeurs
A Private chat is pay-per-minute (at whichever rate you, or the site, sets for it). On most sites, the member that initiates the private show with you will be the only one who can chat with you, but other "voyeurs" may watch the video portion of it, also paying a lesser pay-per-minute rate. On some sites, your private shows may be recorded by the site, and either offered to the person who paid for the private, or also offered for an additional fee to other members. Make sure to check with the site you're on about their policies and practices regarding privates being recorded.
On Streamate, a "Premium" show is actually more like a Group show. See below for info on Group shows.
Private with no voyeurs
A Private chat without the ability for others to watch or participate in. Normally these are offered at a higher per-minute rate since there is no ability to receive extra income from voyeurs, but it varies by site.
On Streamate, the 'exclusive' chat is a non-voyeur private show.
A Group chat is a pay-per-minute show where multiple members are included and are all paying the same rate and have the same level of participation.
On Streamate, Premium chat is a type of group show. One member must initiate the Premium chat, but after that any member may enter or exit as they wish, with every member paying the standard rate that the model chooses for Premium shows. The show continues until the model ends it or the last member leaves Premium chat.
This show type is specific to Streamate. It is similar to a Group chat, but pledges of $ are given before the start of the show instead of there being a pay-per-minute rate. There is a flat fee to join (though more can be given if the member decides to) and a specific amount of time set for the show. Once the time is up, the show automatically ends, and the $ is collected from the member and given to the model. If the show ends before the predetermined time is up, or if the show never begins, the members are never charged and the model receives nothing.