Wordings that are attached to a Treaty but that are not themselves legally binding. They may help with the interpretation of legally binding articles but the EU Court is not bound to follow the interpretation.
There are 65 Declarations annexed to the Lisbon Treaty. After the Irish No to the Lisbon Treaty a new declaration concering Ireland was adopted by the European summit in June 2009. It was not attached to the treaty but will eventually be inserted in a later treaty of enlargement.
The declaration states explicitly that it does not change anything in the Lisbon Treaty. See Irish declaration.
Notes
When all member states agree, declarations are a "common political position".
Declarations made by just one member state (unilateral declarations) will normally highlight areas of disagreement (i.e. other member states having refused to agree on the point the Declaration covers). If the other countries had agreed to a unilateral declaration it would have become a common declaration.
Protocols are legally binding additions to Treaty articles.
Opt-out declarations (derogations) are always interpreted in a narrow way.
Earlier on a lot of secret unpublished declarations were adoptedby the member states in Coreper in relation to EU legislation. On the basis of these, EU laws or treaty provisions could basically be changed even though the declarations were not legally binding and not applicable by the European Court of Justice.
These secret declarations still exist and may have policy effect. They have never been published.