Under U.S. copyright law, you have a copyright as soon as you make a fixed, tangible copy of your original work. Copyright protection extends to all original ideas and expressive elements of the work. In the case of a book, expressive elements include plot devices, setting, tension and characters. In the case of a song, they can include the lyrics, the arrangement, the notes, even the voice.
If you register your copyright, you get additional rights, including the statutory damages and the award of attorney fees should you hire an attorney to enforce your rights against an infringer. Both of these rights give you tools to protect your work, both ideas and expressive elements, against unauthorized copying or alterations. When two works are similar, or one work incorporates pieces of another, infringement generally turns on the question of whether the whole or parts of the second work are "substantially similar" to the original or whether the whole or parts of the original work has been substantially altered.
Under the "fair use" doctrine of U.S. copyright laws, others may use limited portions of your work, such as short quotations, without your permission for purposes including book reviews, classroom lessons or news reports. U.S. copyright law also permits some trivial similarities between two works. If there is substantial similarity, however, in whole or part, between the works, then the copying is generally impermissible. If the second work is not substantially similar and has been substantially altered, then the copying may be deemed permissible. Keep in mind, what constitutes originality, substantial similarity and substantial alteration is not always clear.
Substantial alteration means generally that the original, creative expression of your work has been changed enough to create a new, valuable, non-infringing work. Substantial alteration goes beyond how much of the second work is original or how much of the original has been copied. Think about what makes your song or movie original, memorable and distinguishable. Has that been changed in the new work or not? In the case of music, the most original or “meritorious” parts can be quite brief, so infringement may be found even where only a small part is copied without any substantial alteration.