There are many problem archives online, each with its own characteristics. Project Euler has math problems where answers are submitted via plaintext, TopCoder has its own flavor of problems with an ancient Java applet interface (although it has a new, still very primitive, web version in beta).

Compared to other problem archives, the Kattis Problem Archive has a very high ratio of quality/quantity. Also, the average problem statement on Kattis is more interesting than the average problem statement on TopCoder/Codeforces (e.g. "Fox Ciel just got this sequence of integers as a birthday present from his grandma, ...".)

On Euler you're asked for the answer, not an implementation solving the problem. This means that you can solve the problem with an implementation that takes days to run or require some manual steps. On Kattis you submit source code to the system, so the runtime limit will be on the order of seconds (definitely not days).

Topcoder has "more interface". With Kattis you would typically code in whatever editor/IDE you have and prefer. Also, Topcoder asks you to implement a function, Kattis asks you to write an entire program. The most important (?) difference being that the latter includes I/O.

Im pretty sure that this is a public version of the platform that has been running at KTH (royal institute of technology) for years. It was used as a test suite as well as performance test for assignments there for the CS school.