OLTP (online transaction processing) records many small read and write operations with transactional integrity, such as sales or payments. OLAP (online analytical processing) queries and aggregates large volumes of historical data for reports and trends.
In DP-900 it is a guaranteed question: OLTP uses normalized models and ACID transactions; OLAP uses denormalized data, often in a star schema inside a data warehouse or lakehouse.
OLTP vs. OLAP is a concept that can show up on the exam. Check the full guide to see it in context.