The Street
The Barksdale Organization (the largest drug dealing gang in W. Baltimore)
Avon Randolph Barksdale: the kingpin 
Russell "Stringer" Bell: the CEO
the lieutenants
Roland "Wee-Bey" Brice
Anton "Stinkum" Artis
Maurice Levy: lawyer of the Barksdale Organization
Marquis "Bird" Hilton
Ronald "Ronnie Moe" Watkins
D'Angelo "Dee" Barksdale (nephew of Avon Barksdale)
the pawns
Preston "Bodie" Broadus
Wallace
Malik "Poot" Carr
Omar Little:the lone wolf (robs drug dealers, including Avon's organization)
Reginald "Bubbles" Cousins: the "snitch" (works as a Confidential Informant for Shakima Greggs of the Baltimore PD)
Joseph "Proposition Joe" Stewart: the kingpin (he and his organization control narcotics trafficking in E. Baltimore)
Ervin Burrell: commissioner of Baltimore PD
the Homicide Unit
William Rawls: the commander
James McNulty
William "Bunk" Moreland
the Narcotics Unit
Jay Landsman: squad sergeant
Raymond Foerster: the commander and major
Shakima "Kima" Greggs
Cedric Daniels: shift lieutenant
Thomas "Herk" Hauk
Ellis Carver
Major Crimes Unit (cross-unit detail established to build cases against high profile drug dealers of the Barksdale Organization)
Cedric Daniels, James McNulty, Shakima "Kima" Greggs, Thomas "Herk" Hauk, Ellis Carver
Lester Freamon
Patrick Mahon
Roland "Prezbo" Pryzbylewski
Leander Sydnor
Augustus Polk
Michael Santangelo
Judge Daniel Phelan (pushed the Baltimore PD to investigate the Barksdale Organization)
Terrance "Fitz" Fitzhugh (McNulty's friend at FBI's Baltimore Field Office)
Rhonda Pearlman (judge in the Circuit Court for Baltimore City, the chief prosecutor for the Major Crimes Unit)
The School
Edward J. Tilghman Middle School
Roland "Prez" Pryzbylewski (teacher)
Howard "Bunny" Colvin (Field Researcher at the University of Maryland School of Social Work)
Howard "Bunny" Colvin: commander of the Western District in the Baltimore PD until retirement
Duquan "Dukie" Weems
Namond Brice (son of Roland "Wee-Bey" Brice)
Michael Lee
Randy Wagstaff