Called the "Path To Prosperity," it is a proposed budget for the federal government that includes massive tax cuts and tax simplification, with spending cuts to major government entitlements like Medicare, Medicaid, and Social Security. 
Here are the big things:
  • Ryan's plan calls for major cuts in government spending (on Medicare and Medicaid) and reduced tax revenue.
  • The plan does not balance the budget for at least 20 years
  • Ryan would redraw personal income taxes into two brackets--25 percent and 10 percent. The plan doesn't exactly specify yet what income would qualify you for the higher bracket. 
  • There would also be a massive drop in corporate taxes from 35 percent to 25 percent.  And there would be hardly any tax deductions either.
  • Big cuts to Medicare, Medicaid, and (implied) Social Security, along with the elimination of Obamacare.
  • Existing Medicare users would keep their current plan (a bribe for existing seniors).
  • In the future, the plan would offer a government subsidy that Americans could use to buy private insurance. The idea is that competition among insurers would keep costs down.
There are hundreds of little details that you can find in the plan itself. 
The plan is similar to Romney's own plan for the economy, which calls for cutting tax rates 20% across the board and trimming government spending expect for defense.
Ryan's plan has been blasted for being a cynical attempt to shame the Democrats without having any realistic chance of being passed. It also has been criticized for proposing to fix America's budget problem by cutting benefits for those who can least afford them.
But Ryan deserves credit for having done what the rest of Congress refuses to do, which is acknowledge that U.S. government spending is on an unsustainable path--and offering specific ways of addressing that.
Other Congressmen, and Romney, pay lip service to the need to cut the budget deficit, but they lack the balls to actually come out and offer a specific plan. Ryan did that.