There are four types of legislation: bills, joint resolutions, concurrent resolutions and simple resolutions. Any of these may be introduced in either chamber. They are identified as follows:
View definitions for the types of legislation from GPO.
Legislation may be changed or amended as it makes its way through the legislative process. You will sometimes see different versions of legislation in THOMAS, and these versions are identified by extensions to the bill number. For instance, H.R. 2301.ih indicates the version of H.R. 2301 as it was introduced in the House. A list and definition of common extensions may be found at GPO.
Source: Thomas FAQs: http://thomas.loc.gov/home/faqlist.html#3