Simple question > Assign to-do task

In my workflow, I need to send travel custodians an email when a request a approved so they travel custodians can reserve tickets. I am TOTALLY confused by how content type plays role here. I thought when you assign a to do, an entry is made to some task type list. Please suggest how this work?

2nd question: Is there a way to determine when Custodian(s) is/r done with the task so that request item status can be changed to complete.

Thanks