Alpaca offers an API that allows your trading algorithms and applications to access real-time price, fundamentals, place orders and manage your portfolio, in either REST (pull) or streaming (push) style. For details, please read API docs.

API v2 - Documentation | Alpaca
Alpaca API lets you build and trade with real-time market data for free.

A list of API features include Account API, Orders API, Positions API, Assets API, Watchlist API, Calendar API, Clock API, Account Configurations API, Account Activities API, Portfolio History API, Market Data API.