Question

what is the difference between field level an record level validation rule

Answer 1

Field level controls how the data can be enter in a database field.
Record level controls how the data can be enter in a record or row of data in a database table....

Answer 2

Field level validation rule controls how the data can be entered in a database field(column)
Record level controls how the data can be entered in a record or row of data in a database table. 
example: suppose you have a field called 'student_phone_number'
then the validation rule wud be 'only 10 digits allowed.' this will be valid for all entries in this field.
if you have a field called 'marks'
then the record validation rule wud be 'less than 100 & more than -1'.