Today, the Silver Star, Navy Cross, Distinguished Service Cross, and Air Force Cross are known as awards that recognize heroic actions by service members in combat.
But they were created on the heels of serious controversy over other awards.
During the Civil War, Congress created the Medal of Honor to recognize valor. But some of the awards were seen as questionable by some. Perhaps the most egregious of these was the case of the 27th Maine Regiment.
According to HomeofHeroes.com, the 864 men of this regiment were awarded the medal en masse due to a poorly-worded order by Secretary of War Edwin Stanton and a bureaucratic snafu.
So, in 1917, there was an effort to clean up the mess that had been created. A total 911 Medals of Honor were revoked, including those from the 27th Maine. But there was also an effort to make sure that the Medal of Honor would not be so frivolously awarded in the future, while still recognizing gallantry in action.
As America entered World War I, it was obvious there would be acts of valor. So Congress created the Distinguished Service Cross and Silver Citation Star (Which would later become the Silver Star Medal) in 1918, and the Navy Cross in 1919 to address valor that didn't rise to the level of the Medal of Honor. It was the start of the "Pyramid of Honor," which now has a host of decorations to recognize servicemen (and women) for valor or for other meritorious actions.
So, what sort of courageous actions warrant which medal? Perhaps one indicator of today's standards can come from the sample citations in SECNAV Instruction 1650.1H.
Historically, though, it should be noted that in the Vietnam War, Randy "Duke" Cunningham received the Navy Cross for making ace (an Air University bio reports that he was nominated for the Medal of Honor for his actions on May 10, 1972).
Or, one can look at the actions of Leigh Ann Hester to get a good idea of what would warrant a Silver Star.