(Conference) Proceeding(s): a collection of academic papers that are gathered into a book post-conference (papers are reviewed before publication). It is possible that an academic journal will use the term "proceedings" as part of its title even though it does not consist of conference papers. e.g. 2008 International Conference on Computational Intelligence for Modelling Control & Automation (available through IEEE Xplore).
Transaction: a serial publication, a term used in the title of a number of scientific journals e.g. IEEE Transactions on Aerospace and Electronic Systems (available through IEEE Xplore)
Preprint: "Something printed and often distributed in partial or preliminary form in advance of official publication: a preprint of a scientific article." (2011). In The American Heritage dictionary of the English language. Retrieved from http://search.credoreference.com/content/entry/hmdictenglang/preprint/0
e.g. Search for preprints in E-Print Network, arXiv.org etc.
White Paper: an authoritative report to educate, in government typically a policy or position paper, in business/IT industry an informational document to educate customers re: a new product or service by explaining the technology e.g. "U.S. Energy Infrastructure Investment: Large-Scale Integrated Smart Grid Solutions ..." by the Power Systems Engineering Research Center (PSERC) at Arizona State University and "Technology White Paper on Solar Energy Potential on the U.S. Outer Continental Shelf" by U.S. Dept. of Interior, Minerals Management Service, and Renewable Energy and Alternate Use Program.
Technical Report: a report on new developments or results from scientific/technical research, often funded by corporations or government departments e.g. "Recommendations for Design, Construction, and Maintenance of Bridge Approach Slabs: Synthesis Report" from National Technical Reports Library (NTRL)
Examples of other sites which contain technical reports, list provided by Standford Libraries.
Standard: a document outlining guidelines/rules for activites, established by consensus and approved by an organization/association e.g. a technical committee e.g. IEEE Recommended Practice for Radar Cross-Section Test Procedures (available in IEEE Xplore)
Examples of standards associations (provided by MIT).
Letters: a serial publication e.g. Micro & nano letters (available in IEEE Xplore from 2006 forward)
Virtual Journals: online collections of citations with links to the article in the original parent journal e.g. SPIE Letters Virtual Journal