Possible values for addAttributeToFilter

Collections in Magento have the method addAttributeToFilter. It filters entities.
This method takes an attribute code and a condition.

$products = Mage::getModel('catalog/product')->getCollection();
$products->addAttributeToFilter('sku', 'test-sku');
$products->load();

Where 'test-sku' - value of condition.
If value of condition is integer or string - exact value will be filtered with 'eq' condition).
If value of condition is array - one of the following structures is expected:

array("from" => $fromValue, "to" => $toValue)
array("eq" => $equalValue)
array("neq" => $notEqualValue)
array("like" => $likeValue)
array("in" => array($inValues))
array("nin" => array($notInValues))
array("notnull" => $valueIsNotNull)
array("null" => $valueIsNull)
array("moreq" => $moreOrEqualValue)
array("gt" => $greaterValue)
array("lt" => $lessValue)
array("gteq" => $greaterOrEqualValue)
array("lteq" => $lessOrEqualValue)
array("finset" => $valueInSet)
array("regexp" => $regularExpression)
array("seq" => $stringValue)
array("sneq" => $stringValue)