Itzik Ben-Gan explains Inline Table-Valued Functions:
Compared to the previously covered named table expressions, iTVFs resemble mostly views. Like views, iTVFs are created as a permanent object in the database, and therefore are reusable by users who have permissions to interact with them. The main advantage iTVFs have compared to views is the fact that they support input parameters. So, the easiest way to describe an iTVF is as a parameterized view, although technically you create it with a CREATE FUNCTION statement and not with a CREATE VIEW statement.
It’s important not to confuse iTVFs with multi-statement table-valued functions (MSTVFs). The former is an inlinable named table expression based on a single query similar to a view and is the focus of this article. The latter is a programmatic module that returns a table variable as its output, with multi-statement flow in its body whose purpose is to fill the returned table variable with data.
Now that we have that sorted, click through to see examples and dive into performance ramifications.