All men have an individual soul/ spirit. This is called atman. This atman can unite whit the Brahman. The future existence off the atman will be determined by his karma.

Moksha (Enlightenment) :
The ability to free one self from the eternal circle of life and death and merge with Brahman.

Samsara :
This is the eternal circle of life and death.

Nirvana :
Nirvana is a state in which one is free of suffering. Nirvana can be reached by reaching Moksha. Nirvana is the salvation of the laws of karma, when suffering (dukkha) will come to his end.