This is most important method among the Post Hartree-Fock methods.This is a numerical technique which applied on medium size molecules.

Types:

S - for single excitations (shortened to singles in coupled-cluster terminology)

D - for double excitations (doubles)

T - for triple excitations (triples)

Q - for quadruple excitations (quadruples)

see http://en.wikipedia.org/wiki/Coupled_cluster

This is most important method among the Post Hartree-Fock methods.This is a numerical technique which applied on medium size molecules.

Types:

S - for single excitations (shortened to singles in coupled-cluster terminology)

D - for double excitations (doubles)

T - for triple excitations (triples)

Q - for quadruple excitations (quadruples)

see http://en.wikipedia.org/wiki/Coupled_cluster

CCSD(T) explicitly call as - Coupled Cluster single-double and perturbative triple. Its different from CCSDT which is  called Coupled Cluster Single-double and triple. The Term in round brackets indicate that this calculated based on perturbation theory . So in CCSD(T), the triple excitation is calculated by using perturbation theory but in CCSDT, the triple excitation term is calculated as usual way. 

CCSDt is also used for the calculation of the reaction constant and the transition states.