Basically, Crimea surrendered to Ukraine in 1992 when they inserted a clause into their constitution (under Ukrainian threats of war), declaring that Crimea was part of Ukraine, though autonomous and independent within it. A few weeks later the Ukrainian Rada abolished the 1992 constitution and declared that future constitutions had to be approved by the Ukrainian parliament. In 1994 the Crimean parliament voted to restore the 1992 constitution again, and then the Crimean President decided to work with the Crimean parliament to rewrite it to guarantee more autonomy. The Ukrainian Rada responded by not only abolishing the 1992 constitution a second time, but by eliminating the presidency in Crimea along with it and placing the Crimea under decree-based direct administration under the Ukrainian president himself. The Crimean parliament, now under the thumb of the President of Ukraine (the president of Ukraine was literally running Crimea by fiat), ended up rewriting the constitution in a way favorable to Ukraine, with major edits placed by the Ukrainian Rada. That constitution went into effect in 1998, and has governed Crimea ever since.
The differences between the two are huge. It's like the difference between Puerto Rico and Hawaii to the U.S. Hawaii is an integral U.S. state, and it's leadership is entirely subject to the U.S. government. Puerto Rico is a sovereign U.S. territory. Independent, but American. Its residents hold citizenship in both countries, but it's still generally regarded as "American soil". Most importantly, Puerto Rico has a LOT more legislative freedom than Hawaii does, and can vote to leave the U.S. at any time if it chooses.