This is the two bedroom apartment across the street of Monica's Apartment. For the first four seasons and the first half of the fifth season of Friends, it was occupied by Ugly Naked Guy. When the guys see him packing in The One Where Everybody Finds Out, Ross decides to fill in an application and try to bribe the renter with a basket of mini-muffins. Ultimately, Ross obtains the apartment in this episode when he drinks tea with Ugly Naked Guy and sharing his mini-muffins. He lives in this apartment for the rest of the series.
Things don't start in the best way for Ross in the new apartment - he spots Monica and Chandler fooling around (he's still unaware of their relationship), he shows signs of uncontrolled rage in front of his former boss in spotting Monica and Chandler, which costs him the chance to return at work at the museum; and the house-warming party he holds when he moves in is a total failure to which only he and Phoebe attend after being kicked out of another party for the retiring handyman, Ross having declined to contribute to the handyman's retirement fund because he had only just moved in (in The One With The Girl Who Hits Joey).
Even though the guys often express positive criticism about Ross' apartment, they rarely spend time there, with the exception of Rachel (especially from being pregnant with Emma to the rest of the series), although Phoebe also lived there for a few days to give Chandler and Monica some time to themselves after they became engaged. However, the apartment is shown a lot more than his old apartments. Also, unlike Monica's apartment, none of the other guys have keys to Ross' place. This becomes a problem for him in The One With Ross's Step Forward when he gives girlfriend Mona his key, as they contemplating moving in together (Although the key argument eventually proves that the two of them are moving too fast).