# What is RL? A short recap [[what-is-rl]]

In RL, we build an agent that can **make smart decisions**. For instance, an agent that **learns to play a video game.** Or a trading agent that **learns to maximize its benefits** by deciding on **what stocks to buy and when to sell.**

To make intelligent decisions, our agent will learn from the environment by **interacting with it through trial and error** and receiving rewards (positive or negative) **as unique feedback.**

Its goal **is to maximize its expected cumulative reward** (because of the reward hypothesis).

**The agent's decision-making process is called the policy π:** given a state, a policy will output an action or a probability distribution over actions. That is, given an observation of the environment, a policy will provide an action (or multiple probabilities for each action) that the agent should take.

**Our goal is to find an optimal policy π* **, aka., a policy that leads to the best expected cumulative reward.

And to find this optimal policy (hence solving the RL problem), there **are two main types of RL methods**:

- *Policy-based methods*: **Train the policy directly** to learn which action to take given a state.
- *Value-based methods*: **Train a value function** to learn **which state is more valuable** and use this value function **to take the action that leads to it.**

And in this unit, **we'll dive deeper into the value-based methods.**

