They are different industry standards defined to rate an 
organisation as per their performance and other factors.

CMM-Capability maturity model

PCMM- People Capability muturity model.

The CMM i.e capability maturity model defines processes to 
be followed at each level (by experience) to achieve the 
desired result. 
SEI - Software Engineering Institue that certifies the 
various companies by the help of their representives in 
various countries.
PCMM-Peoples CMM i.e there are about 199 people practices 
followed across the various companies. i.e based on the 
feed back of the people about the company,if we make 
required changes that yeild better results and healthy 
environment for the people working in it. ex; Black box, e-
mails,etc.
Six Sigma in brief is the standard process in moving 
towards total bug free Software(ex) like 99.9999%.Though it 
is similar to CMM5, it is v.rigid in some aspects like 
rejecting the defect SW component and producing a new one. 
No compromise in quality. Motorola company is one such Six 
sigma company.

CMM: is Capability Maturity Model which is commonly known 
as CMMi in IT industry.
SEI: is Softwage Engineering Institute, of carnegie Mellon 
University, Michigan who has developed this model besides 
People Capability Maturity Model, which has its transition 
partners across globe for implmentation of the Model and 
Lead assessors for carrying out the People CMM assessment.

PCMM: is People Capability Maturity Model developed by SEI, 
which mainly concentrates on People Processes which mainly 
aligns the business in attracting and retaining the best 
talent in the industry.  The model is known world wide for 
its roubustness and replicability.  There are 5 maturity 
levels, 22 processes with 89 goals (People CMM V.2)