Financial statement level means a risk pervasive to the financial statement as a whole and casts doubt on the true and fair view.
Assertion level risk is the risk that a director assertion over an FSA is inappropriate. It has two components, inherent risk and control risk. E.g. PPE – existence (does it actually exist – asset verification), completeness (are all PPE accounted for and in the right period – subsequent bank review and review of minutes)
From memory, there should be a number of posts on this topic here, so give it a search.