The Room of Spirit and Time is a room that is located in Kami's Lookout. The room is first introduced in the Piccolo Daimo Saga, when Goku trained with Kami in the anime, but, in the manga, the room was introduced during the Cell Game, as a room to train to get stronger. One day on Earth is equivalent to one year in the room. That is why the room is there. So, you can train for one day, when you could have trained for a year. Why haven't they just trained and trained in the room you ask? It's simple. Anyone that enters that room more than twice will be locked in nothingness for all eternity. The people that have entered that room that we have seen are:

Goku - Twice
Piccolo - Twice
Vegeta - Twice
Gohan - Once
Mirai no Trunks - Twice
Goten - Once
Chibi no Trunks - Once
Super Buu - Once
Krillin - Once
(Fusion) Gotenks - Once

Goku came in as a kid and as an adult. He only trained for about 5 months as a kid, then, couldn't stand the extreme heat to extreme coldness. But, as an adult and Super Saiyan, he could take the 10 times gravity and weather conditions. Vegeta came in twice, he trained so hard, he became very close to USSJ, and then went back to train once more to surpass Goku. Gohan came in as a kid, and trained so hard, he became a Super Saiyan in that room. See, as I see it, Goku and Gohan share a bond with turning Super Saiyan. When Goku turned Super Saiyan, only Gohan and Freeza saw. But, Freeza died, so only Gohan saw. Then, Goku only saw Gohan turn Super Saiyan. See how that all comes out? That's just my theory. Mirai no Trunks came in twice, and achieved USSJ in there, while, his father came very close while training with him. He went in again, to train and become stronger to kill Cell. Goten came in once, while training with  Trunks. They were training, so they could fight, "the really powerful guy", or we know him as Super Buu. They trained so hard together, but I'll lead on to that with Trunks. Trunks went in with Goten, as I said, to train against Super Buu. Now, as they were training in fusion, they achieved something not even Vegeta could. They became Super Saiyan 3 Gotenks. Piccolo lead Super Buu to Goten and  Trunks, while they prepared, and, fought in the Room of Spirit and Time. In that time, Super Buu found out it was a trick, and made a portal to get out, leaving Piccolo, Goten, and Trunks in there. Piccolo trained once in the Cell Saga, only trying to get stronger, but not too much. Another time was when he walked in with Super Buu; to see Goten and Chibi no Trunks kill Super Buu. However, he got trapped in the room, and had to escape with Gotenks. Now, you may are wondering, Krillin? He went in the room? Well, he did. He went in while the door was open, to tell Goten and Trunks to get up for Super Buu. Then, he ran out. That is all the people we get to see go in the Room of Spirit and Time. Was there anyone else that went in? I don't know. That's all we know about the Room of Spirit and Time.