Measuring a person's ability to speak a language is not an exact science. There are too many factors at play. But if exams and tests are to mean anything, an attempt must be made to define levels of competence. Historically there have been many different ways of measuring somebody's language ability and many different scales to indicate level. No world-wide system of measurement or rating exists, though CEF is a Europe-wide system that measures ability in many languages including English. In general, all systems move from absolute beginner (no knowledge of the language) to advanced (equivalent to native-speaker), but the number of levels varies from system to system: some have 5 levels, some have 7 levels, some have 10 levels, and so on.
These pages show what is probably the closest thing to an international standard that we have (the CEF), as well as the ALTE "descriptors" that describe what a person "can do" at a given level, and a table showing alignment between various exams and the CEF. Finally, you are welcome to try the simple test to get an idea of your current level (Written Comprehension only).