What is a 'multi-part identifier' and why can't it be bound?

A multipart identifier is any description of a field or table that contains multiple parts - for instance MyTable.SomeRow - if it can't be bound that means there's something wrong with it - either you've got a simple typo, or a confusion between table and column. It can also be caused by using reserved words in your table or field names and not surrounding them with []. It can also be caused by not including all of the required columns in the target table.

Something like redgate sql prompt is brilliant for avoiding having to manually type these (it even auto-completes joins based on foreign keys), but isn't free. SQL server 2008 supports intellisense out of the box, although it isn't quite as complete as the redgate version.

Actually sometimes when you are updating one table from another table's data, I think one of the common issues that cause this error, is when you use your table abbreviations incorrectly or when they are not needed. The correct statement is below:

Update Table1
Set SomeField = t2.SomeFieldValue 
From Table1 t1 
Inner Join Table2 as t2
    On t1.ID = t2.ID

Notice that SomeField column from Table1 doesn't have the t1 qualifier as t1.SomeField but is just SomeField.

If one tries to update it by specifying t1.SomeField the statement will return the multi-part error that you have noticed.

It's probably a typo. Look for the places in your code where you call [schema].[TableName] (basically anywhere you reference a field) and make sure everything is spelled correctly.

Personally, I try to avoid this by using aliases for all my tables. It helps tremendously when you can shorten a long table name to an acronym of it's description (i.e. WorkOrderParts -> WOP), and also makes your query more readable.

Edit: As an added bonus, you'll save TONS of keystrokes when all you have to type is a three or four-letter alias vs. the schema, table, and field names all together.