Heroku is a cloud application platform, a PaaS (platform as a service): it runs a developer's application without the developer managing servers or operating systems. Heroku is a product of Salesforce, Inc.
A developer deploys with a git push or a Docker image, and Heroku runs the app in dynos: lightweight, isolated containers on a managed runtime that also patches the operating system and manages TLS certificates. An app scales by adding dynos or by moving to larger ones, and Performance dynos can scale automatically. Heroku officially supports Python, .NET, Java, Node.js, Go, PHP, Ruby, Scala and Clojure. Its managed data services include Heroku Postgres (a PostgreSQL database) and Heroku Key-Value Store (a managed Redis service). The Elements marketplace adds add-ons and buildpacks, the scripts that prepare an app for its language, and newer services provide managed AI inference and agents.
Heroku is a hosted service billed per dyno per month, from small Eco dynos that sleep after 30 minutes without traffic to dedicated Performance dynos; databases are priced separately.