Each advertising company uses the same terms in slightly different ways; more on, often a company offers to its clients services than span across multiple theoretical roles.
With just a couple of questions you can find out which Resove service best fits your needs, and what you normally do.
Publishers have one or more website or app on which they want to put ads. Resove will push advertisers' ads to these spaces, and will then pay for how many visits, clicks and actions these ads generate.
Advertisers have some ads that they want to be shown around the web. In order to do so, they start advertising campaigns, selecting both the target audience and what they prefer to be billed for (e.g. impressions, clicks or actions).
An agency is able manage other publishers and advertisers accounts on their behalf. This type of account could also be seen as a "super account".
Networks have both spaces (e.g. websites, apps) and ads. They use Resove as an ad server, to both manage the quirks of advertising campaigns (e.g. rotations, quotas, etc.) and buy/sell their surplus.