A civil marriage is simply one where the marriage ceremony has a government or civil official performing the ceremony. It takes place without any religious affiliation and meets the legal requirements of the state or location of the wedding.
In the UK, a civil registrar ceremony cannot include hymns, religious readings or prayers and the marriage must take place at a registered or licensed venue to be legally valid. Many private premises are licensed to hold civil ceremonies. As well as each party to the marriage signing the register, signatures of two witnesses are also required.