What is the difference between gerim, nochrim and zarim? Both halachicly and denotatively.
It depends on context, but in general there are two types of Gerim:
1) A Ger Tzedek (Righteous convert). That is a convert in the traditional sense.
2) A Ger Toshev (Dwelling convert). That category doesn't formally exist today, but the concept of it is a non-Jew who accepts the Seven Mitzvos of Noach. They have permission to live in Israel.
Typical usage of the word "Ger" denotes the former.
A Nochri would be a non-Jew. In some contexts it may seek to distinguish between a Ger Toshev and otherwise, but it means a foreigner.
A Zar (stranger) almost always means the term for a Jew who is not a Cohen.