Hyperbolic Time Chamber

The Hyperbolic Time Chamber (The Room of Spirit and Time") is a place located in Kami's Lookout. One year inside the chamber is the equivalent to one day on the outside. There is a higher gravity level than Earth's. A step over the threshold of the training area brings one immediately into ten times Earth's gravity (the same as that of Planet Vegeta, Planet Zoon, and King Kai's planet), the air gets denser, and the temperature fluctuates, the deeper one goes into the training area.
The Hyperbolic Time Chamber has an entrance at its center, located in a central building with two side wings with housing, food supplies, bathing quarters and sleeping quarters. There is no night or day in the Room, but the surroundings remain a constant bright iridescent white aurora. Its reflective floor is of undefined area, and the Room's boundaries are thought to stretch to infinity in all directions even though it appears to have a definite atmosphere, limiting its size. Goku explains to Gohan during their visit to the chamber that the air grows increasingly dense in the recesses beyond the plaza, which can be threatening as the Hyperbolic Time Chamber has no definite boundaries. The Hyperbolic Time Chamber represents a separate dimension from that of Earth. 
The time dilation that occurs in the Hyperbolic Time Chamber is one day of real time is equivalent to one year in the Chamber, or 1 minute of real time is approximately six hours in the Chamber. There are dual giant hourglasses of emerald sand adorning the sides of the building that count down a year within the chamber. A clock on the dome roof of the main part of the building tells the applicable time in the real world. 
This essentially allows for someone entering the Hyperbolic Time Chamber to get one year worth of intense training in a single day. 
The gravity in the Hyperbolic Time Chamber living area is described as being "different", though it seems to not be much higher than Earth's gravity.
The wooden door entrance is a portal between the Earth and the Hyperbolic Time Chamber, and is the only way to travel between the two dimensions without ripping holes in the dimensional walls. 
The Chamber can only be entered for two days (or two years) at a time, if anybody attempts to stay longer, they will not be able to exit, leaving them in nothingness and not allowing anyone else to use the door. Also, there are only enough provisions to accommodate two people for the full year. This is commonly misinterpreted as a two-person limit overall, but it should be noted that in the Buu Saga four people in total entered through the main door.