ISNULL is a function which is verify the data is null or not if null then it will replace the value which is assigned in the function.NULL is default values if no data present.

Commit is a TCL  command which is  used to make  database 
transaction permanent.
once the data is commited it can't be rollbacked.

ISNULL is boolean to check value null or not whereas
NULL is a default value that we can assign to a records