While certain components of each level are standardized, each reiki master will teach them a little differently. There are typically three (sometimes four) levels of reiki training—each one is centered on "attunement" (a ceremony of empowerment), education, and practice.
Attunement is what makes reiki unique from other forms of healing touch and energy work. This ceremony, called Reiju (Ray-joo) in Japanese, is performed by the reiki master during each level to open and expand the main energy channels of the student's body, allowing universal energy, also known as Qi/Chi/Ki, to flow more freely and deeply through them. It tends to clear blockages in the body and can lead to significant energy changes within the individual. Many will undergo an intense period of self-growth following their attunements. You can think of the attunement process like a transfer of energy: The master is passing energy off to the student, who will then possess it for life.
While it's possible to receive Level 1 and Level 2 training over the course of the weekend (for a few hundred dollars, depending on where you live), you can also find trainings that are stretched out over weeks or months. Typically, the reiki master degree is more intensive and takes months to receive.
Here is a more detailed breakdown of the three levels of reiki and each of the attunements: