Because Kusama is built as a testnet for Polkadot, the technical functionality of both networks are the same. However, the founder of Kusama said that Kusama is like the unknown frontier, where everything is undiscovered but offers more opportunity for evolution. Here are key differences between the two:
Kusama network will add new referendums for token holders to participate in voting more than Polkadot. The frequency is around 4 times more. Therefore, Kusama will significantly have more updates than its cousin.
The minimum number of token required for participation in Kusama network is significantly less than Polkadot. That is why Kusama has more Validators or Parachain higher than Polkadot. Kusama also has a program called 1000 Validators to further boost the number of participants.
While Polkadot is mainly built for applications that require stability and security like finance or corporate usage, Kusama is known for its speed for development. You can say that Kusama is like a Laboratory where new applications or technologies are being tested.