Salesforce Connect lets you expose data from external systems (SAP, Oracle, REST endpoints, etc.) as if they were Salesforce objects, without copying data into Salesforce.
External Objects are the result — they look like custom objects (MyExternal__x suffix) but every read goes to the external system in real time via a configured External Data Source.
apex List<MyExternal__x> rows = [SELECT Id, ExternalId__c, Name FROM MyExternal__x];
When the SOQL fires, Salesforce makes an API call to the external system and returns rows. Caching is configurable.
Use cases: avoid duplicating millions of rows of legacy data; show real-time inventory or pricing from an ERP; provide read-only views into a data warehouse without ingesting it.
Limitations: limited write support; performance depends on external system; 100-record join limit; some features (reports, formulas) work but with caveats.
Modern alternative: Data Cloud for richer multi-system unification.