How the Star Trek universe has evolved through the basic premise of each series.
The original Star Trek series, which had a three year run in the 1960s, set the tone for what Star Trek was going to be about. It was a show that took place entirely in space. They rarely went back to Earth. I think they went back to Earth a couple of times in strange time-travel fashions.
Fifteen years later, Gene Roddenberry was asked to do it again. He wisely chose to do it in another century, and went ahead one century from the 23rd century to the 24th. He also wisely decided to not try to find another James T Kirk. We ended up picking a bald, British, Shakespearean actor to play the role.
The Next Generation had certain things about it that were a century beyond the original series. We had holodecks for amusement's sake and also for research sake. The ship was far more advanced and far more comfortable - we had families and kids on the ship. It was, a slightly more futuristic era - Kirk and those people were all dead, but it turned out that Spock wasn't. It was a long time after the original series.
Then, about three years into that series, when Gene Roddenberry was still alive but not at all well, Paramount came to me and said they wanted to do another series which would overlap for a couple of years with The Next Generation. I asked Michael Pillar to join me in creating that series, which turned out to be Deep Space 9.
We realised that if it was going to be on the air simultaneously with The Next Generation for two years, it couldn't take place on a starship. It would be ridiculous to have two starships out there. So, we decided, instead, to have it take place on a Cardassian space station, off in the middle of nowhere, right by a worm hole that was open to another part of the galaxy.