US Copyright Law/Copyrightable Works
To be copyrightable, a work must be an original work of authorship that is fixed in a tangible medium of expression. These requirements are often shortened to creativity and fixation.
An original work of authorship can be just about anything. 17 U.S.C. § 102(a) lists eight basic kinds, by way of example:
- Literary works - Any work expressed in letters, numbers, or symbols, regardless of medium. This Wikibook is a literary work under the Copyright Act. Computer source code is also considered to be a literary work. If a literary work is fictional, its characters, plot, structure and other devices can be copyrighted along with the actual text, but only to the extent that they are original and expressive.
- Musical works - Original musical compositions. Actual performances of music are covered as a different type of work, a "sound recording." 17 U.S.C. § 115 provides that anyone may create a sound recording of any composition, so long as they pay that composer a set licensing fee.
- Dramatic works - Distinguished from literary works in that they direct a performance through written instructions.
- Choreographic works - Added in the 1976 Act. They must be "fixed," either through notation or video recording.
- Pictorial, graphic and sculptural works - Any two-dimensional or three-dimensional art work may be copyrighted as a PGS work. These works are constrained by the "useful article doctrine" described below, which serves to keep copyright and patent law divided.
- Audiovisual works - Includes films, television programs and any other work that combines audio and visual components.
- Sound recordings - Any magnetically or digitally recorded audio work (except the accompanying audio to a visual work, which is treated as part of the audiovisual work). Unlike musical works, there is no statutory licensing of sound recordings. While anyone may produce a rendition of an artist's composition by paying him the statutory fee, they may not reproduce his recordings without his express permission.
- Architectural works - Added in the 1976 Act. Copyright in architectural works only extends to aesthetics. If a design element is "functionally required," it is not protected by copyright. See H.R. Rep. No. 101-735 (1990) and "Useful article doctrine," below.
Idea vs. expression
Copyright does not protect ideas, whether they are fact, opinion or fantasy. It only protects the expression of those ideas.
Facts vs. Compilations
Copyright does not protect recording of facts that are "common property" and contain no "original authorship". Some examples are standard calendars, tape measures and rulers, height and weight charts, and lists or tables taken from public documents or other common sources. For example a recipe which contains only a list of ingredients is not copyrightable, but a recipe contains elements of original authorship such as explanations, instructions, and illustrations can be copyrighted. 
On the other hand compilations of facts such as dictionaries and even telephone directories can receive copyright protection. But the facts themselves cannot be copyrighted—only the expression of those facts can be copyrighted. This means that many database providers may not be able to rely on copyright to protect their data from being republished: they must rely on licenses and other elements of contract law. In other cases, the law of unfair competition can prevent one business from using another's compilations of information in an unfair way. See Intl. News Serv. v. Associated Press, 248 U.S. 215 (1918) (holding that INS could not re-report AP news stories on the West Coast for commercial advantage).
In the realm of software, source code and object code are copyrightable, but other elements of a program may not be. User interfaces, for instance, are generally not copyrightable: see Apple Computer v. Microsoft, 35 F.3d 1435 (9th Cir. 1994) (icons and desktop appearance of Windows not copyrightable); Lotus Development v. Borland Intl., 49 F.3d 807 (1st Cir. 1995) (menu structure of Lotus 1-2-3 not copyrightable).
Useful article doctrine
Many pictorial, graphic and sculptural works are also patentable as useful inventions. If the creative component of a work cannot be separated from its functional component, it is considered an industrial design and it cannot be copyrighted. This is known as conceptual separability. One well-known example is the Brandir Ribbon bicycle rack, which adopted an artistic design quite different from previous bicycle racks. Because the creativity of the design was inseparable from its functionality, the rack was deemed protectable only through patent law. Brandir Intl. v. Cascade Pacific Lumber, 834 F.2d 1142 (2d Cir. 1987). For more on patents, see the Wikibook on US Patent Law.
Works of the U.S. federal government are not copyrighted. 17 U.S.C. § 105; see also Wheaton v. Peters, 33 U.S. 591 (1834) (official reporter of the Supreme Court unable to protect annotations of Supreme Court opinions).
State statutes and court opinions are generally considered to be public domain as well, because of a public policy interest in allowing citizens unrestricted access to the law. See Nash v. Lathrop, 6 N.E. 559 (Mass. 1886) (state statutes held to be public domain); Veeck v. Southern Bldg. Code Cong. Intl., 293 F.3d 791 (5th Cir. 2002) (model statutes enter public domain when enacted).
However, other state government works can be copyrighted when the creator of the work needs an economic incentive; see Co. of Suffolk v. First Am. Real Estate Solutions, 261 F.3d 179 (2d Cir. 2001) (county allowed to hold copyright in tax maps).
Fixation requires that the work be recorded in a form that can be perceived directly or by means of a device "now known or later developed." Writing, drawing, sculpting, recording and making notations all count as fixation. Improvisational and/or unrecorded performances of music, dance or drama are likely to be creative but not fixed. Something that is typed on a computer but never saved to a permanent storage medium might also be unfixed (but once the work is saved, it is fixed). Oddly, ice sculptures are not adequately fixed to warrant copyright protection, but RAM copies of data are adequately fixed.