A Standard Field ships with the object — Account Name, Industry, Owner, Created Date, and so on. You can't delete a standard field. You can usually edit its label, help text, and picklist values, and on most you can change field-level security and page-layout placement, but the API name is fixed.
A Custom Field is one you create. Its API name always ends with __c (for example Annual_Revenue_Forecast__c). You choose the data type — Text, Number, Currency, Date, Picklist, Formula, Roll-Up Summary, Lookup, Master-Detail, Geolocation, Encrypted Text, and so on. You can edit, delete, change field-level security, and reference custom fields in formulas, validation rules, flows, reports, and Apex.
Two practical differences matter in interviews. First, deleting a custom field doesn't fully remove it for 15 days — it sits in a deletion queue and can be restored. Second, field limits are real: each object has a cap on the number of custom fields (typically around 800), and certain field types like Roll-Up Summary, Master-Detail, and Long Text have their own sub-limits. If you keep adding custom fields casually, you can hit those limits and have to refactor.