Types of records in appian

Appian consists 3 types of records

1.Entity-backed record

2.Process-backed record

3.Service-backed record

check the below documentation

docs.appian.com/.../Records_Tutorial.html

Priyanka gave you the only sensefull differentation.
If you are targeting for "customer", "buildings", "orders", applications or whatever you can think about.
They are not premade. You have to make/maintain them. That is part of the "low code palatform vs premade platform system"- journey ;)

The other posters have mentioned one of the differences: the source of your record type. The other meaningful difference between different recrord types is whether they have sync enabled or not. Record types with sync enabled enable you to cache your data in Appian to improve performance. There's also a bunch of features that build on using the data sync, such as related records and custom record fields. If you use a record type without sync enabled, each time you access the record type it makes a request to the source system.

Here's some documentation on record data sync: https://docs.appian.com/suite/help/latest/records-data-sync.html

Also one minor terminology clarification - the "record type" is a design object in Appian, and you as a low-code developer can create as many record types as you like. A "record" refers to a single row or item of that type. So the record type could be "Customer" and the record could be "ACME Corporation".