Germany is one of the most prosperous among EU countries with high standards of living, developed infrastructure and great cultural capacities.
There are 4 ways to stay in Germany: getting a visa, a settlement permit, permanent EC residence and naturalisation.
If your reason for leaving Germany is only temporary (for educational, trading, humanitarian or familial reasons), you can apply for a limited settlement permit (Aufenthaltsgenehmigung).
If you have been living in Germany on a settlement residence permit for five uninterrupted years, and satisfy certain other conditions (secure livelihood, fluent German, clear criminal record), then you are entitled to stay living in Germany indefinitely on the unlimited settlement permit (Niederlassungserlaubnis).
Later you will be eligible to apply for German citizenship.
Visa, residence permit & citizenship