RISKS
Risks are events, which if occur, will have either a negative or positive impact on the project. The risk management process should include: risk identification, risk qualification, risk quantification, risk response, and risk monitoring and control. (see www.pmi.org (Guide to PMBOK for more detailed defintiions).
ASSUMPTIONS:
Assumptions are things you believe to be true or real and must validate. (e.g. you assume you have the right staff for the project, which can be validated prior to the project). However, it would not be wise to assume you will not have turnover, since that is a risk event which will have a negative impact on the project and must be mitigated.
EXCLUSIONS:
Explicit statements which indicate whatwill NOT be provided in the scope of the project. These should not be assumptions, but statements of fact.
ISSUES:
Points of concern which must be evaluated and resolved. They may translate into risks once they've been identified and quantified as to their significance.