View Execution Plan for Multi Statement TVF, and why Inline TVF better

When you execute a Multi-statement table valued function (TVF), you will see an execution plan that looks something like this:

The optimizer always estimates a Multi-statement TVF has only a single row return, which is mostly not the truth. In order to see the detailed plan inside the Multi-statement TVF, you can run the following SQL statement to get it from the plan cache:
SELECT  deqp.query_plan, dest.text,
SUBSTRING(dest.text, (deqs.statement_start_offset / 2) + 1,
(deqs.statement_end_offset - deqs.statement_start_offset)
/ 2 + 1) AS actualstatement, deqs.creation_time
FROM    sys.dm_exec_query_stats AS deqs
CROSS APPLY sys.dm_exec_query_plan(deqs.plan_handle) AS deqp
CROSS APPLY sys.dm_exec_sql_text(deqs.sql_handle) AS dest
WHERE   deqp.objectid = OBJECT_ID('YourFunctionName');
The result will look like this:

Multi-statement TVF can be bad for performance due to the fact that it's treated like a table except there are no statistics available for SQL Server to base a good execution plan on - so it will estimate the function as returning a very small number of rows. If it returns a larger number of rows, then the plan generated could be a lot less than optimal.
Let's compare the showplan difference between Inline TVF and Multi-statement TVF. Below are two TVFs, one Inline and another Multi-statement:
CREATE FUNCTION udfInline
(   
    @id int
)
RETURNS TABLE
AS
RETURN
    SELECT * FROM Account
GO
CREATE FUNCTION udf_Multi
(
    @id int
)
RETURNS
@result TABLE
(
    AccountId int, UtcOffset int
)
AS
BEGIN
    INSERT @result SELECT * FROM Account
    RETURN
END
GO
The Inline TVF execution plan look like this:

For Inline TVF, the estimated number of rows are quite accurate. The actual underlying tables get brought into the main query and a better execution plan can be generated based on proper statistics. You'll notice that in this case, the execution plan will NOT have a mention of the function at all as it's basically just merged the function into the main query.

The Multi statement TVF execution plan look like this:

For Multi-statement TVF, SQL Server made the estimation as only one row. There's a great reference on it on MSDN: "But if you use multi-statement TVF, it’s treated as just like another table. Because there is no statistics available, SQL Server has to make some assumptions and in general provide low estimate. If your TVF returns only a few rows, it will be fine. But if you intend to populate the TVF with thousands of rows and if this TVF is joined with other tables, inefficient plan can result from low cardinality estimate."