Severity of a bug describes the impact of a bug.
Blocker: Blocks development and/or testing work
Critical: crashes, loss of data, severe memory leak
Major: major loss of function
Minor: minor loss of function, or other problem where easy workaround is present
Trivial: cosmetic problem like misspelled words or misaligned text
Enhancement: Request for enhancement
Priority describes the importance and order in which a bug should be fixed. This field is utilized by the programmers/engineers to prioritize their work to be done. The available priorities range from P1 (most important) to P5 (least important.)
Example: Any look and feel issues like spelling mistake on an UI is less severe, but if the product is going for beta testing, priority of bug to fix is higher.
Similarly any exception coming on a particular operation has high severity, but that module is not to be delivered right now, so this bug can have low priority