Bill - The primary form of legislative measure used to propose law. Depending on the chamber of origin, bills begin with a designation of either H.R. or S.
Simple Resolution - A form of legislative measure introduced and potentially acted upon by only one congressional chamber and used for the regulation of business only within the chamber of origin. Depending on the chamber of origin, they begin with a designation of either H.Res. or S.Res.
Joint Resolution - A form of legislative measure used to propose changes in law, or to propose an amendment to the U.S. Constitution. Depending on the chamber of origin, they begin with a designation of either H.J.Res. or S.J.Res.
Concurrent Resolution - A form of legislative measure used for the regulation of business within both chambers of Congress, not for proposing changes in law. Depending on the chamber of origin, they begin with a designation of either H.Con.Res. or S.Con.Res.
If you wish to study legislation in draft form before being introduced and assigned a bill number you will need to access archival collections.
Before legislation can be proposed and assiged a "bill" number, someone has to draft a bill. Many people and organization in and outside the government try to influence the legislative process by drafting bills. These include: Congressional; State Legislature; Executive Branch; advocacy groups and other organization; and individual people.
Here you will find some prominate organizaitons which actively draft legislation to influence the process, but keep in mind that any organizaiton can be promoting draft legislation: