The policy chooses the actions to be taken in a given situation and can be categorized as deterministic or stochastic.

A deterministic policy is denoted as a_t = µ(st), while a stochastic policy can be denoted as a_t ~ π(.|s_t), where the tilde symbol (~) means has distribution. Stochastic policies are used when it is better to consider an action distribution; for example, when it is preferable to inject a noisy action into the system.

Generally, stochastic policies can be categorical or Gaussian. The former case is similar to a classification problem and is computed as a softmax function across the categories. In the latter case, the actions are sampled from a Gaussian distribution, described by a mean and a standard deviation (or variance). These parameters can also be functions of states. 

When using parameterized policies, we'll define them with the letter θ. For example, in the case of a deterministic policy, it would be written as µ_θ (s_t).