The fifth book - called 'The Order of the Pheonix' - tells us all about the Order. 'The Order' (OotP) is a group of magicals who oppose Voldemort. The 'OotP are like resistance fighters.

The group has been about for at least 15 years (we haven't been told when they formed). We are shown a picture of 'The Original Order of the Pheonix'. The pic includes
Alastor Moody,
Dumbledore.
Degalus Diggle,
Marlene McKinnon (killed "they got her whole family"),
Frank and Alice Longbottom ( he was an Auror. She was tortured too to try to get Frank to tell Deatheaters (LeStrange's, Crouch Jr and others) where Voldemort was hiding after he met Harry/ after V'd fallen) ,
Emmeline Vance,
Benjy Fenwick (dead - "in bits"),
Edgar Bones ("they got him and his family.. he was a great wizard" The Bones family is of course still relevant 'cause Susan Bones is in Harry's year at Hoggers and her Aunt is Ammelia Bones who procided over Harry's hearing in the 5th),
Sturgis Podmore,
Caradoc Dearborn ("never found his body"),
Elphias Doge,
Fabian and Gideon Prewett (Molly Weasley's bro's. Took 5 Deatheaters to bring them down; "they fought like heroes"),
Dorcas Meadows (V killed personally),
Aberforth Dumbledore (Dumb's bro. Famous for "being prosecuted for practising inapproprite charms on a goat" ??? Where's he???),
Remus Lupin,
Hagrid Sirius Black,
Peter Petigrew
Lily and James Potter.

'Order of the Pheonix' Members since the above:
Minerva mcGonagall (though not in the original photo, we get the impression she has been in the Order a long time - there when Potters killed)
Kingsley Shackebolt (Auror)
Nymphoradora Tonks ( Auror)
Hestia Jones
Molly and Arthur Weasley
Mundungus Fletcher (Thiefa nd all round nice spy)
Doreen Figg ( Lives on Privett Dr., Squib and kneazle breeder)
Sniverus Snape ( Don't know how long h's been a member - "What has he done that makes dumb trust him?" is the question that HAS to be answered!)
Olympie Maxime (Beaubatum's Headmistress. Went searching with hagrid for giants on Dumb's orders)

These are the ones I could remember/ find. Hope we can find more.