Basic attack

An autoattack (also called standard, normal, or basic attack) is the attack performed by a champion when right-clicking an enemy unit. Autoattacks are also performed by an idle champion when an enemy unit ventures too close to it, or when the "attack move" function is used (default shift + right-click).

Champion
When using attack-move, the champion stops to attack any hostile that gets in their way en-route to a destination. If a champion has an attack target, they will continue to attack it if in range and will move closer to it when out of range, until either a new target is specified, a cancel order is issued (pressing the S key or by specifying another move order), or they loses vision of the target.

Vision
Autoattacking when hiding in the brush will reveal the attacking champion for a few seconds. Autoattacking while in stealth mode will break the stealth.

Damage
Autoattacks always deal physical damage (though certain items, abilities or passives may cause them to deal additional magical damage on top of their base physical damage). Damage from autoattacks benefit from attack damage and can be reduced by armor.

When autoattacking a turret, the damage is based on the base attack damage added to either the bonus attack damage or 40% of a champion's ability power, depending on which one is higher.

Attack rate
The rate at which a champion attacks is called attack speed and is capped at 2.5; This mean that a champion can attack at most 5 times in a 2 seconds period.

Some abilities refresh the attack timer and allow a champion to attack again immediately if they use the ability after their autoattack.

A list of abilities that refresh the attacktimer:

Critical strike
Critical strikes deal 200% of the standard attack damage, though this amount can be increased through skills, runes, masteries and items. When critically striking using an autoattack modifying ability, only your attack damage will be multiplied, not the ability damage. (Note that not all of these abilities can critically strike.)