So what is roaming anyway? Roaming is a strategy which involves a champion going from lane to lane ganking and keeping pressure on enemy champions. The roamer sacrifices levels and farm in favor of getting their solos fed and the enemy team denied. An effective roamer can terrorize a map and keep the enemy team paranoid.
Footage taken from League of Legends, a free to download MOBA created by Riot Games. www.leagueoflegends.com