Collectables are entities that are placed within a game world that do not provide in-game benefits to the player when found or earned. (such as additional score, lives, etc.). They have no instrinsic value within the game but are extrinsically interesting to the player. (for aesthetic and/or showoff value)
This is not the same as objects that are collected but do serve a purpose within the game. For example, collecting shines in Super Mario Sunshine allows the player to obtain access to previously unavailable areas of the game. They refer to objects that have no real gameplay value.
Legend of Zelda:Link's Awakening
In The Legend of Zelda: Links Awakening, the player character meets a photographer. In certain specific locations in the game the photographer makes an appearance and takes a picture of the player character. These pictures are kept in an album where they may be perused by the player. They are completely unnecessary in order to finish the game.
In Shenmue, the player can spend money to get a random egg from a machine. Inside each egg is a miniature action figure. There is a set of these figures that can be collected.
Castlevania:Harmony of Dissonance
In Castlevania: Harmony of Dissonance, the player may collect various pieces of furniture scattered around the castle and deposit them in a blank room set up for that purpose. Collecting all the furniture gives nothing but a tastefully decorated room when finished, and has no bearing on the rest of the quest.
In Maximo, the player can, at certain locations in certain levels, buy a new pair of boxer shorts. The boxer shorts come in different designs and the player can only possess one pair at a time. This is a weak example because the player cant have more than one pair of boxer shorts. However, the shorts are completely inconsequential to the completion of the game. It is also interesting to note that in the sequel to Maximo (Maximo 2: Army of Zin), the player can also change boxer shorts but this time they confer special abilities. Thus, in Maximo 2 the boxer shorts are no longer considered Collectables.
In Earthbound there are a few worthless objects that do nothing when used, such as the Ruler and Protractor, which when used in battle simply give the message "*character* can now find the length/angle of things easily!" and waste a turn. They can be sold, thus giving them a slight value in the game world and making this a weak example, but not for much. Another candidate is the Insignificant Item, which while doing nothing by itself can be given away to get a relatively useful item in return. The object holds no value by itself, but since it can be used to get something useful does not really count as a Collectible.