"The House and Senate must pass the same bill or joint resolution in precisely the same form before it can be presented to the President. Once both houses have passed the same measure, they can resolve their differences over the text of that measure either through an exchange of amendments between the houses or through the creation of a conference committee. The House and Senate each have an opportunity to amend the other chamber's amendments to a bill; thus, there can be House amendments to Senate amendments to House amendments to a Senate bill. If either chamber accepts the other's amendments, the legislative process is complete. Alternatively, each house may reach the stage of disagreement at any time by insisting on its own position or by disagreeing to the position of the other chamber. Having decided to disagree, they then typically agree to create a conference committee to propose a single negotiated settlement of all their differences. Conference committees generally are free to conduct their negotiations as they choose, but they are to address only the matters on which the House and Senate have disagreed. Moreover, they are to propose settlements that represent compromises between the positions of the two houses. When they have completed their work, they submit a conference report and joint explanatory statement, and the House and Senate vote on accepting the report without amendments. Only after the two houses have reached complete agreement on all provisions of a bill can it be sent to the President for his approval or veto."
CRS Report for Congress, 96-708