What is Encrypted with Field-Level Encryption?

In contrast to Classic Encryption, which uses a custom field type in the Salesforce data model, Shield Platform Field-Level Encryption makes more fields, files, and data elements available for encryption with every release.

With FLE you can encrypt most standard and custom fields. You can encrypt fields using either a probabilistic or deterministic encryption scheme. Probabilistic is more secure, but there's a tradeoff for filtering and sorting with certain types of data. Deterministic is more flexible, but it is less secure, and if you use the case-insensitive variant, it creates an extra sorting field.

FLE (Probabilistic) shares the tenant secret used for Files and Attachments Encryption and Chatter.

FLE (Deterministic) uses a dedicated tenant secret.

For the complete list of fields that you can encrypt with Shield Platform Encryption, see What Can You Encrypt? in Salesforce Help.