The core rules of What's O.L.D. is N.E.W. deals with three broad eras. While these can be broken down into numerous smaller ages, this is not necessary to play the game.

Each of the three standalone books, O.L.D., N.O.W., and N.E.W., presents a single era in detail.

Archaic.  This era covers the stone age, all the way through medieval times, up to the rennaissance and introduction of early firearms. It features swords, elves, and dragons.  It is covered in detail in O.L.D. which focuses on medieval fantasy.

Modern. This era broadly covers the 20th and early 21st century. It features martial arts action heroes, tricked out vehicles, superspies, and ninjas. It is covered in detail in N.O.W. which focuses on 80s-style contemporary action.

Future.  This era covers anything beyond the present day. It features starships, laser swords, blasters, and psionics.  It is covered in detail in N.E.W. which focuses on galactic exploration, colonization, and conflict.

A game setting can be based on one of these three eras, or it can mix and match to put wizards on starships.  In terms of what technology to include, the default rule (unless the setting says otherwise) is that any technology from an earlier era is available in a later era at double the price, and that technology from a later era is not available in a previous era.  These items are designated archaic, modern, and future in this rules reference document.

The three supernatural elements - magic, chi, and psionics, can each be included in any of these eras. 

Genre

Genre indicates the scale of realism to fantasy.

Out Of Time

Out Of Time is an optional rule. It has two effects:

1. Characters operating equipment not from their own era (archaic, modern, future) suffer -1d6 to attribute checks.
2. If equipment from multiple eras is in use, then archaic equipment counts as one quality level worse (to a minimum of standard), while future equipment counts as one quality level better (to a maximum of artisanal). Modern equipment is unaffected.