1. What is the difference between waterfall model and software process model?

Waterfall model is one type of software process model.
So there is no difference between the two but they share a
parent-child relationship.
A software process model is used in software engineering and
it encompasses a set of activities, methods, actions, tasks
and products.

A waterfall models is a linear model that suggests a
systematic, sequential approach to develop a software
starting from communication with the client and passing
through planning, modeling, construction to the deployment,
delivery and feedback.

2. Explain the difference between a bug and a defect?

While similar to each other and tracked using the same tools,
a bug is considered to be an error in
satisfying requirements while a defect is missing a
requirement altogether.