The Developer Console is a browser-based IDE built into Salesforce. Provides Apex code editing, SOQL Query Editor, anonymous Apex execution, Apex Test Runner, debug logs viewer, heap dumps, governor-limit dashboard, file explorer, and workspace tabs.
Modern reality: the Developer Console is increasingly basic. Most professional Apex/LWC development uses Salesforce Extensions for VS Code (or JetBrains Illuminated Cloud) which offer better editing, source control integration, and debugging.
When to still use Dev Console: quick anonymous Apex fix (data hotfix), inspecting a debug log without local tooling, ad-hoc SOQL, quick test runs without leaving Salesforce.
When to use VS Code instead: real development (multi-file editing, source control), refactoring across files, local unit-test runs (faster feedback), LWC development (Dev Console doesn't support LWC bundle structure well).