Gliese 581 is a rather unremarkable red dwarf star in the Libra constellation... Were it not for one small fact. It has planets orbiting it, three that we know of. Even that would not make it remarkable. The fact that one of them is the first potentially habitable planet found thus far however does make it exceptional.
Gliese 581 is 20.4 light years (about 120 trillion miles) from Earth. It is so dim that you can not see it without a telescope (Apparent magnitude +10.55). It is one third the mass of the sun and because of this lower stellar mass it's surface temperature is far cooler than the sun's. Gliese 581 is so small it is only 31 times as large as the Earth. The three planets orbiting this star have been given the inventive names Gliese 581b, Gliese 581c and Gliese 581d.
The "c" planet is the first potentially habitable planet that we have found. It has 5 Earth masses and a 170 lbs man like myself would feel like they weighed 272 lbs on its surface. A year on this planet lasts only 12.9 days and scientists believe that there are no periodic days or nights on this planet. One side always faces the star and the other faces away. Its orbit is tidally locked to the star, a common condition for planets of low-mass stars.
There is still a lot to learn. Scientists have no idea whether the planet is a terrestrial planet (rocks and dirt) or an ice ball covered with water. While they are pretty certain that it has an atmosphere, they are uncertain as to the composition of it.
Gleise 581c orbits at a range of only .071 AU (about 6.5 million miles) from the star. That means that the disk of the star in the planet's sky would appear 20 times larger than the Moon appears on Earth.
The other two planets (b & d) are both larger gas giants. Gliese 581b orbits the star at a distance of .041 AU (3.8 million miles), completing one orbital circuit in a mere 5.4 days and is around 16 Earth masses. Gliese 581d (about 8 Earth masses) orbits at a distance of .25 AU (23 million miles), taking 83.6 days to complete one orbit.
Unfortunately it is rather unlikely that we'll be sending a probe to the Gliese 581 star system any time soon. The fastest spacecraft to date was the Helios probes placed in orbit around the sun. They clocked a blistering 150,000 MPH and even at that speed a space craft would take over 91,000 years to reach Gliese 581.