You only need to register with CIS if you directly employ staff on a non-permanent basis, but who are not bona fide sub-contractors. Confused? You will be.
If you employ, e.g. a plasterer full time they are PAYE and you deduct tax and NI to pay them net, but you are also liable for employers NIC's and pension as well (unless they have opted out). If work slows down it can be harder to let them go if on this employed status. As building is mostly on set contracts it isn't always efficient to work this way, hence the CIS scheme. You can still employ the plasterer but on a more casual basis and he / she gets few employment rights. You have to register with HMRC as a CIS employer and deduct money from their gross which you pay to HMRC (prevents them paying no tax at all).
The plasterer then has to sort out their own tax affairs each year and claim back any over paid tax. If you are running a development you may have other builders working for you as sub-contractors. They are there on a contract basis and invoicing you for works.
They will sort out their own tax and NI - it is not your problem. Many firms run a combination of all 3 systems simultaneously, myself included. My 15 hard core lads are all full time employed and I have regular work for them all year. I bring in people on CIS to ramp up numbers when I have larger projects, but want control over exactly what they do and where. I then bring in subbies for particular work on a fixed time / cost basis.