Hi there, summoner!

What is a DPS champion?
One of the best starter champions in League of Legends is Ashe. The first thing you need to know is, that Ashe is a DPS champion and DPS means 'Damage Per Second'. So a DPS champions is characters that is supposed to deal tons of damaage. In-Game that means that you have to buy a lot of damage, attack speed and lifesteal items.

What is a tank champion?
A tank champion is a character, that has to be strong in team battles. This means that if a team has a tank, they'll propably send the tank into the fight first to take all the aggro and aggro means 'attack focus'. To be a tank you'll have to buy a lot of health and armor. Armor do you more resistance to basic attacks, but not against magic damage. To be more resistence against magic damage you'll have to buy magic resist.

What to do against tank champions?
So why I'm telling you what a tank is, when you're a DPS champion, is because you don't wanna start attacking to tank, because he's restisting your attacks and the other players will kill you before you even get close to kill the tank champion. So only attack a tank champion last in a team fight or if you're alone on a lane with a tank, then you can peak the tank champion. What is a lane? A lane is one of the roads where the minions goes. What is peaking? Peaking is when you don't get damaged and just trying to get the champion losing some health, but be care because peaking can absolutely get you killed.
So to kill a tank, you've to buy armor penetration. What does armor penetration do? Armor penetration makes you deal damage right trough their armor, but don't start focus armor penetration, unless all your opponents are tank champions.