There is wide range of things that can be copyrighted: literary, dramatic, musical, artistic, pictorial, graphic, sculptural, motion pictures and other audiovisual works, sound recordings, and architectural works. To be copyrighted, it must be fixed in a tangible form. Even choreography can be copyrighted if filmed or notated (thus being “tangible”). Copyright covers both published and unpublished works.
Facts and ideas are separate from copyright. Only their particular expression or fixed form can be copyrighted. Folklore, traditional knowledge, and even the English language are all examples of things that are not copyrighted. However, they can be used to create works that are copyrightable - for example, a modern retelling of a fairy tale can be copyrighted by its author (for example, Disney’s movie “Beauty and the Beast”).
Some common examples of copyrighted material that as you may use in doing research: books, articles, software, CDs, DVDs, art works, musical works, etc.
Copyright length can be a complicated matter because the copyright laws have changed several times during the last century. Works created since 1978 are automatically protected, whether published or not, with a term ending 70 years after the death of the author (“life plus 70”). Works for hire are covered for 120 after creation years or 95 years after publication, depending on which is shorter.
Prior to 1978, works had to be published or registered with the copyright office to be covered. With the current law, works created before 1978 that were unpublished or unregistered are now covered with the “life plus 70” rule. Works published or registered between 1923 and 1978 have a maximum copyright length of 95 years - as long as the copyright was renewed in the 28th year, as the previous law gave a copyright length of 28 years plus one renewal. Which means, it is not always clear if a work created before 1978 is still under copyright or not. Generally, it is best to assume that the work is copyrighted.
Copyrightable works published before 1923 are in public domain.
For more on the duration of copyright: