Aeris was born at the Icicle Inn (North Continent);
her biological parents being Professor Gast & the Ancient Ilfanda.

Gast was hiding Ilfanda from Shinra at a building in Icicle Inn;
they soon fell in love and in time give birth to Aeris.

The Shinra find out where Gast was hiding; they kill Gast, & Ilfanda runs off with Aeris.

Ilfanda & Aeris end up in Midgar (wonder how they got all the way there...).

Unfortunately, the Shinra catch up with them, and kill Ilfanda at the Midgar train station.

A lady named Elmyra was there at the moment, and Ilfanda tells Elmyra to take care of Aeris. From them on, Elmyra raised Aeris; and Aeris was still being tracked down by Shinra; mainly the Turks.

Since Ilfanda was an Ancient, that makes Aeris an Ancient, too.

It is believed that the Ancients (who call themselves the Cetra) can lead everyone to the legendary Promised Land; that's why Shinra want to capture Aeris so badly.

Since the Promised Land is supposedly peaceful and stuff, they figure that there's lots of Mako there.