What DML operations are available in Apex and what does upsert do?

DML operations: insert (creates new records), update (modifies existing records), upsert (inserts if no match found, updates if match found), delete (moves to Recycle Bin), undelete (restores from Recycle Bin), merge (merges up to 3 records of the same type, deleting duplicates). Upsert: Uses a specified external ID field (or the record Id) to determine if a record exists. If a match is found, the record is updated; otherwise it is inserted. Syntax: upsert records ExternalId__c; Use Database.upsert() with allOrNone=false for partial success. Upsert is ideal for data integration scenarios where you cannot guarantee whether a record already exists.