Process trees are a process modelling language for which inception and early development, like many other modelling languages, Wil van der Aalst played a pivotal role. The hierarchical nature of process trees provides many theoretical guarantees on which one can build. Therefore, unsurprisingly, process trees have often been used and applied in process mining techniques developed in recent years. Yet, a concise overview, combined with a solid formal foundation, is lacking. Therefore, in this paper, we provide a unified definition of process tree operators as defined in literature and discuss their applications and relation to Petri nets. It should not come as a surprise to the reader that the usage of process trees or related modelling formalisms, as well as other works relevant to this work, yielded a total of 60 references.