I have read about windows workflow foundation where people use this to model a business process, why not to use UML?
According to some answers, the workflow can be my domain?
What other tools are equivalent to the WF?
Workflow foundation is an executable workflow - the framework includes an engine that executes the workflow.
It allows you to write parts of your workflow logic in code.
I suggest reading this developers introduction on MSDN.
UML doesn't do either thing - you can't use code as part of your UML or execute your UML workflows.
UML is intended as a documentation and communication tool.
Workflow Foundation is a software tool that is used for modelling and executing workflows.
Update, regarding suitability for modelling the domain.
Since the core of Workflow Foundation is to model workflows, that is what you should use it for. If you core domain is not workflows, you should model it in a more traditional manner. You can use Workflow Foundation to model the workflows of your model.