It is one of the Saronic Gulf Islands
Poros Town is the main town on the island.
About an hour by ferry or 45 minutes by fast ferry from Pireas (Athens' Port)
Poros is a fairly small island (it is actually two islands, though with the vast amount of landfill that has taken place, you may not notice the join at the "canal" adjacent the naval training base). A moderately fit person can walk from the town centre to the monastery in approximately an hour. However, there are two buses which run from their common terminus at the port to the monastery and to Neorion. The fares are moderate. They do not, however, tend to run very late into the evening as demand is fairly low at that time. Their timing is fairly predictable except at lunchtime when the schoolchildren break up and buses can occasionally be kept waiting to pick them up.
There are also a number of grey taxis on the island - also reasonably cheap to use. Expect to pay about 6 Euros plus tip from the town centre (that is, the port) to the monastery. Mercifully, the island is too small to support a population of rogue taxis.
Driving one's own car within the town itself is not really recommended as the seafront road is narrow and one way only except for mopeds and motorcycles. It is frequently constricted by pick-ups and vans delivering to tavernas and shops (unavoidable) and the thoughtless who will park their car anywhere to avoid walking a few extra yards (eminently avoidable).
Poros is an extremely safe place to stay - crime is minimal and in 20 years of visiting, the editor of this section has never seen an ugly situation or heard voices raised in anger. The biggest danger to your safety is likely to be yourself if you hire a motorcycle or moped on the assumption that you can learn to ride it "as you go".