What is PlantUML?

PlantUML is a pseudo-code way of generating diagrams quickly without worrying about pixel-positioning that traditional drag and drop diagram creation requires. It lets you take pseudo code like this:

@startuml
    Terry -> Jody : Hello()
    Terry <-- Jody : returns "hello yourself"
@enduml

And auto-generate a diagram like this:

PlantUML handles a huge number of diagram types:

Sequence Diagrams
Usecase Diagrams
Class Diagrams
Activity Diagrams
... and so much more - check out the PlantUML site for more details.