Music copyrighting is a tricky and often confusing subject.
As we all know, flat out copying someone's music is a no-no. It is unethical and unlawful. However, there ARE many things we can use from other people's music without actually running in to a Copyright infringement issue. For instance, chord progressions and scales.
Below is a quick tutorial explaining and dispelling some Copyright myths by Richard Stim in his Lynda.com course on the subject.