That's because of the and between the two clauses.
It should be:
SELECT * FROM ClassificationUser.Attribute as attribute
Where (attribute/AttributeName= 'Thread Description' AND attribute/AttributeValue = 'VAM TOP HT')
OR (attribute/AttributeName= 'Thread Size' AND attribute/AttributeValue = '7')
When you put an AND there, both conditions should be true for each record, which is never the case.