Data Import Wizard versus Data Loader
Key Takeaway
Use the Data Import Wizard only when you're loading fewer than 50,000 records into an object the wizard supports, and the import doesn't need to be automated. If any one of those is not true, use Data Loader.
Salesforce gives admins two main tools for loading data from CSV files: the Data Import Wizard and Data Loader. The Data Import Wizard is a guided tool that runs in the browser from Setup. Data Loader is a separate client application for bulk work. The wizard suits smaller, simple imports into common objects. Data Loader handles larger volumes, any object, exports, deletes and automated loads. So the right choice depends on how many records you have, which object they go into and what you need to do with them.
The two tools and where they run
The Data Import Wizard is an in-browser tool. You open it from Setup by entering "Data Import Wizard" in the Quick Find box.
Data Loader is a client application that runs outside Salesforce on Windows or macOS.
Both tools import from CSV files. Data Loader can also read from a database connection.
Data Loader uses API access. Enterprise, Performance, Unlimited and Developer editions have API access. Group and Professional Edition orgs can mass upload data with the Data Import Wizard, but Data Loader is not available in those editions.