You literally can go everywhere in Paris and some near suburbs just by Metro.
Every Metro ticket is good for full trip with as many lane changes you need .
Metro service starts a little after 5 AM to after midnight, or sometimes up to 1-2 AM , depending the line .
Metro is definitely the cheapest and fast way to get around in Paris .
You can buy individual ticket (1.8 €) , book of Ten ticket (14.1 €), weekly pass or monthly pass from clerks or automated machines.
Within the city of paris , Metro tickets are good for buses and RER trains as well .