In Path of Exile, characters start out with level 1 and get stronger as you level up. To finish the main storyline you have to go through 10 acts that are divided in Part I and Part II where you revisit Part I.
In each of the acts you have to complete mandatory quests to progress; there are also optional side-quests that give you great rewards (skill points) but they are considered mandatory for basically every character.
One important quest is the Deal with The Bandits quest in act 2. In this quest, there are 3 bandits that are terrorizing the village that you are asked to take care of. The unique thing about this quest is there are 4 possible rewards that depend on what you do. When you encounter the bandits, you are given the choice of helping him or killing him. If you choose to help one bandit, you have to kill other 2. You can also choose to kill all 3 bandits for a different reward. Here are the possible rewards from this quest:
1) Helping Oak:
- 1% of Life Regenerated per second
- 2% additional Physical Damage Reduction
- 20% increased Physical Damage
2) Helping Alira:
- 5 Mana Regenerated per second
- +20% to Global Critical Strike Multiplier
- +15% to all Elemental Resistances
3) Helping Kraityn:
- 6% increased Attack and Cast Speed
- 3% chance to Dodge Attacks
- 6% increased Movement Speed
4) Killing all the bandits:
- 2 Passive Skill Points
Before doing bandit quest, check out what your build guide recommends as a bandit reward (best build guides should have this information).
As you progress through the acts, the game difficulty will increase not only with monster damage & defense but also with Resistance penalty. After completing Act 5 and Act 10 you will have -30% to all elemental resistances (-60% total), so you will need stronger items to cap the elemental resistances. As soon as you finish Act 10, you have successfully gotten to Path of Exile endgame.