Source
Obligations arising out of the will of the parties are called voluntary, and those imposed by operation of law are called involuntary. Sometimes these are called conventional and obediential. The events giving rise to obligations may be further distinguished into specified categories.
voluntary:
unilateral promise (pollicitatio) - undertaking by promisor only to perform, not requiring the agreement of the beneficiary
contract
quasi-contract
negotiorum gestio - duty to repay someone (gestor) who has managed the affairs or property of another who was unable
solutio indebiti - undue payment or delivery of a thing to another, who is then obligated to return the thing
involuntary:
delicts and quasi-delicts (equivalent to the common-law tort).
unjust enrichment (condictio indebiti)