Copying DVDs is a contentious subject. The Motion Picture Association of America (MPAA) holds the position that copying a DVD for any reason is illegal, and it reserves the right to prosecute you for doing so. The MPAA uses the Digital Millennium Copyright Act (DMCA) to enforce its rights through the courts. In some instances, however, it is legal to copy a DVD.
Copying a DVD for commercial use is illegal under the DMCA. Commercial use includes making copies for sale to others, as well as making copies to display in public. Playing these copies commercially is a violation because the creators of the work are not compensated when they are shown.
Under the DMCA, it is legal to copy a DVD for personal use. Making a copy to use instead of the original, to prevent damage to the original disc is acceptable. Making a copy to store, in case you lose your original copy is also permitted. Making a copy to give to a friend is not considered personal use, and is technically a violation of the DMCA.