A support is someone who makes their teammates stronger, primarily by substantiating the ADC.
A couple of Support champions are:
Sona
Braum
Tresh
Nami
Leona
Sona, Maven Of The Strings. Her abilities are: Hymn Of Valor: Sona sends out two bolts of sound that deal magic damage to the nearest enemies, prioritizing champions.
Aria Of Perseverance: Sona heals herself and the most wounded nearby allied champion, increased by 0.5% for every 1% of target's missing health.
Song Of Celerity: Sona gains bonus movement speed that decays down to Song of Celerity's aura bonus over 3 seconds.
Crescendo: ( ult ) Sona plays an irresistible chord in a line, dealing magic damage to enemy champions and stunning them for 1.5 seconds.
Braum, The Heart Of The Freljord. His abilities are: Winters Bite: Braum propels ice from his shield in the target direction, dealing magic damage to the first enemy hit and slowing them by 70%.
Stand Behind Me: Braum dashes to the target ally, positioning himself between them and the closest enemy champion, gaining bonus armor and magic resistance for 3 seconds.
Unbreakable: Braum raises his shield in the target direction, creating a barrier that intercepts incoming non-turret enemy projectiles and reduces the damage he takes through the shield.
Glacial Fissure: ( ult ) Braum leaps into the air and slams his shield into the ground, creating a fissure in the target direction that deals magic damage to enemies.
Thresh, The Chain Warden. His abilities are: Death Sentence: After a 0.5-second delay, Thresh throws out his scythe in a line and tethers himself to the first enemy hit, dealing them magic damage and stunning them.
Dark Passage: Thresh throws his lantern to the target location where it remains for up to 6 seconds.
Flay: Thresh sweeps his chain in a broad line towards a target direction. Enemies hit take magic damage.
The Box: ( ult ) After a 0.75-second delay, Thresh summons 5 spectral walls around him that last up to 5 seconds. Each wall breaks upon contact with an enemy champion, dealing them magic damage.
Nami, The Tidecaller. Her abilities are: Aqua Prison: Nami sends a bubble towards targeted area, dealing magic damage to all enemies it hits.
Ebb And Flow: Nami unleashes a stream of water onto the target champion or herself which then bounces to nearby unaffected champions up to twice, alternating between allies, including herself, and enemies.
Tidecaller's Blessing: Nami blesses the target allied champion or herself for 6 seconds, causing her target's next three basic attacks to each deal bonus magic damage.
Tidal Wave: Nami summons a tidal wave that surges forth, dealing magic damage to all enemies it hits.