Record Type IDs are typically needed when importing to define the type of record you are importing and by that using/assigning the correct page layout:

To find a record type ID, go to the corresponding object in Setup>Object Manager and switch to "Record Types" in that object's menu, then lastly opening the record type you are interested in.
You will find the ID looking at the URL and find the one ID starting with 012 ->

It always starts with 012, for example an ID could be 012A00000002AraFNB.

On Salesforce Classic:

please go to 

• Accounts: Setup | Customize | Accounts | Record Types 
• Individual Clients & Prospects: Setup | Customize | Accounts | Person Accounts | Record Types 
• Other Contacts: Setup | Customize | Contacts | Record Types 

and click on the record type label. In the address field of your browser, you can now identify the record type id. 

The record type ID is a unique identifier found in Propertybase used to avoid duplicate data and provide

a way to manage your custom business processes. This provides easier administration and page layouts for specific

tasks. It always starts with 012. In this example, the ID is 012A00000002Ara.

All other objects (e.g. Inventory, Offer, ...)

For all other objects, the record type ID can be identified, if you go to 

Setup | App Setup | Create | Objects

Then select the object type, e.g. Inventory and scroll down to the list of available record types. Click on the Record Type Label.

In the address field of your browser, you can now identify the record type id (see section Accounts & Contacts above).