OnLine Transaction Processing [OLTP]
Consider the case where a company makes sell its products to its customers i.e creating sales orders. At a particular moment when any sells happens very few number of records being created in the sales order relevant table(s) or even later you can update the sales order or may event delete the items. That means when any sells happens i.e any transaction processed, few records are being created in the system. That means transaction processing is write oriented and in database terms we can say some INSERT,UPDATE or DELETE operation is happening.
OnLine Analytical Processing [OLAP]
Consider the case where the business wants to know about the sells revenue of all quarters of a financial year( may be previous year(s) ). Now this time no transaction is happening rather than we are just reading the sells information table and displaying the results in form of reports or dashboards. Here exactly the utilisation of the sells historical data happens which involves read operation with very complex SQL queries and involves aggregation operation and so on.
To conclude- OLTProcessings are write intensive operations where INSERT, UPDATE, DELETE SQl commands are frequently used where as OLAProcessings are read intensive operations where SELECT SQL commands with complex aggregation clauses are used.