Bottom lane (Bot) is located literally at the bottom side, and it is as long as Top. However, there is a big difference between Top and Bot. In fact, Bot is a duo-lane, which is also called a shared lane (and 雙人線 in Chinese), while Top is a solo lane. Duo-lane means there are two champions situated at Bot, and the bottom laners usually include one ADC and one Sup.
Most ADCs are marksmen (射手); that is to say, an ADC usually has remote basic attacks (遠程普攻), so ADCs can output their damage far away from the enemy. ADC's tasks are just farming and executing enemy champions in order to gain wealth and then enhance their attack damage (物理攻擊), attack speed (攻擊速度), life steal (普攻吸血), armor penetration (物理防禦穿透), critical strike chance (暴擊機率), movement speed (跑速), and so on. A successful ADC can escape from enemy's spells, basic attacks and ganking.
There are some partial Sup's tasks below:
1. Do not last hit enemy minions or champions because it will make ADC's growth slower.
2. Put wards into important bushes to prevent enemy's ganking.
3. Bother enemy bottom laners, like CC them or cutting down their health.
4. Keep ADC's survival, like helping ADC run away or healing ADC.
In the above picture, red area is Bot, yellow shows the first turrets, and blue are the places Sup should put wards in.
At the end, I want to show you two latest champions, Lucian and Jinx. They are both suitable as ADC.