Congress acts through four kinds of measures. Two can become law; two are the chambers talking to themselves. The letters in front of a number, H.R., S., H.J.Res., tell you which is which.
A bill is the form used for almost all legislation. One that starts in the House is numbered H.R.; one that starts in the Senate is numbered S. A bill becomes law only if both chambers pass it in identical form and the President signs it (or Congress overrides a veto). The number, like H.R. 2150, stays with the measure through every stage.
A joint resolution travels the same path as a bill and carries the same force of law; the choice between them is mostly tradition. Joint resolutions are favored for narrow or temporary matters, such as a single appropriation or a continuing resolution to keep the government funded. The one true exception: a joint resolution proposing a constitutional amendment is not sent to the President at all. It needs a two-thirds vote in each chamber, then goes to the states, where three-quarters must ratify it.
A concurrent resolution must pass both chambers but is never presented to the President and never becomes law. It handles the shared internal business of Congress, like setting the annual budget resolution that guides spending, fixing a date for adjournment, or expressing a non-binding 'sense of Congress.'
A simple resolution is acted on by just one chamber and likewise has no force of law. The House uses an H.Res. to set the terms of debate for a bill (a 'rule') or to create a committee; either chamber uses one to adopt its own internal rules or to state an opinion. It never leaves the chamber that passed it.
Read it in two parts. The first letter is the chamber of origin: H for House, S for Senate. The middle tells you the kind: nothing for a bill, J.Res. for joint, Con.Res. for concurrent, Res. for simple. So S.J.Res. 12 is the twelfth joint resolution introduced in the Senate this Congress.
