You may have come across eins in German: it's the form that's used in a counting sequence (eins, zwei, drei...), whereas otherwise the numeral one is ein.
Раз is much the same thing: it gets used instead of один in the counting sequence. The difference is that it's not mandatory, and, in fact, not standard, but in everyday language, it's virtually never один, два, три but раз, два, три. (Note that the counting-down sequence is still три, два, один; using раз here would sound incorrect.)
Now раз is also a noun, meaning "time" — in the narrow sense of "an instance" (German Mal, French fois, Spanish vez). That's what it will mean most of the times you come across the word, such as in еще раз "one more time".
Раз can also be a colloquial conjunction meaning "since", "now that", "seeing as [something has happened]".