oTree is a Python-based platform used for online tasks. It can be used for behavioral experiments (also for the laboratory), multiplayer strategy games, surveys, and quizzes, especially those that require customized or dynamic functionality (more info here: https://www.otree.org/).
oTree is usually used to conduct online experiments, and you need a platform to run the task and collect the data. Heroku is the most popular one: you can start using it for free and add some extra power ("dynos") for a small cost to make sure the task runs smoothly. If you are using oTree to conduct a lab experiment, you can even use your computer as local server.
The YouTube channel Alkattan Academy created a webinar with an introduction to oTree.
Additional material can be found in the series of videos created by Jonas Frey (University of Oxford).
You can check the platform oTree Hub for additional material to build, launch, and monitor oTree experiments. It also contains a public folder with oTree projects you can test and download.
New experimenters who are not familiar with Python can use oTree Studio instead of a text editor. oTree Studio is a point-and-click interface and it is free to use for small/medium-sized projects.
You find below some videos on oTree Studio.