Hidden object games, also called I-spy games, are screens on which many objects are superimposed, often on a deliberately cluttered background image - if the collection of so many objects isn't cluttered enough in itself - and the player then has to find these objects and click on them, which will be rewarded with a little animation and sound effect, and possibly points or, if the player finds a number of objects quickly one after the other, an extra hint. Hints may at any rate be necessary for the first-time player, because the objects may be hidden in the most devious ways: lined up to match a background colour, shape or pattern, almost completely covered with something else, almost transparent, recoloured to match the background, or shaped out of clouds, shadows or snow. (This is why I always play with the laptop plugged in, so the screen is at maximum brightness; on a dimmed screen, such objects are easy to miss.) Even if the object is in plain sight, its description may throw the player for a loop: "ten" may be the number 10, the word itself, or an X (Roman numeral), while a "dog" may be an actual dog, a stuffed dog toy, a picture of either, the little dog depiction on a "No Dogs Allowed" sign or even the word "DOG". And that's without taking into account words with several meanings, like "pitcher" or "bat", and, a great favourite with hidden object games: "king", "queen", "club", "spade", "knight", that can refer to chess and/or cards. If trying to guess what object such a word refers to isn't enough, the object description can be a riddle, like "waters plants" or "fits on head". The objects the player has to find are generally listed on the left or at the bottom of the screen, generally in writing - by name, or as riddle, or as multiples: five "planes", each of which can be an aeroplane or a paper aeroplane or the tool that shaves planks - but sometimes as silhouettes. Or the description might be a general instruction: everything that fits with a certain theme, or doesn't belong in a room, or has a duplicate of itself in the same scene. Apart from the usual wide screen, there are split screens where the player has to click on the differences; or the objects are invisible until another object (like a "lens") is dragged over them. There are often many more objects hidden in a screen than the player has to find: the object lists can be randomly drawn up whenever the screen is loaded, so the game won't be the same every time it is played.
Though sometimes line drawings, the objects are most often photographic, probably 3D objects that can be rendered in various positions and resized to hide them better. The same object can be used in more than one screen, which can be annoying when, in screen 1, I've looked fruitlessly for a "lobster", and in screen 2 the lobster is staring me in the face, only now I'm looking for a "parasol". I suspect there is a pool of such objects shared by all game companies, because sometimes objects from one game will pop up in another. I also suspect that the game company employees take a devilish pleasure (or just a purely aesthetic one) in arranging these collages.
The object search is always (I haven't seen any exceptions yet) linked to a narrative to give the player a sense of progression. Detective stories, with their emphasis on discovery, are ideal, but the first game of its kind that I played was about souvenir-hunting during a honeymoon. Per stage in the narrative, a game may have several search screens available so that the player, being stuck on one, can switch to another and back before time runs out. Almost every hidden object game has a timer, and every game that I've tried has penalties - usually, losing a hint - for too many wrong clicks.
This is my favourite type of game, so this list will be longest. It strikes
me as the kind of game that's good for children, training their powers of
observation while improving their vocabulary (even adults may come across words
unfamiliar to them - I had no idea what a "dowel" was) yet still challenging and
entertaining for grownups. On me, it has a therapeutic effect, because I am the
kind of idiot to miss things that are staring me straight in the face, and panic
if I don't finish something fast enough.