Trout hide in all kind of places that are a little obvious and sometimes surprising. Overall, there is always one rule to where they position themselves: an optimal spot that always them to expend as little energy as possible while still feeding, yet, it provides cover from predators or quick access to shelter.
Fallen Trees
Trees that have been downed into the water provide excellent structure for trout to take shelter in. Not only do they provide trout a comfortable spot to avoid predators, but it gives them a chance to rest by slowing down currents. In some cases, the tree will even funnel water into a small channel that makes feeding easier.
It can be quite tricky fishing around a fallen tree with branches and limbs protruding in every direction. It's better to pick and choose the areas you'll fish around the tree that present the least amount of risk to snagging up.
Root Structures Along the bank
Stream banks that have been slowly eroded away expose twisted knots of tree roots to the water. Trout will often hide in these arm-thick root structures and await food to drift by before darting out to strike.
Large Rocks
Trout find ideal spots that allow them to rest from the faster currents. Seams and outcroppings along rocks give them this perfect cover.
Rocky Banks Full Of Open Holes
Many streams have large boulders and rocky shores with gaps between rocks or crevices. These gaps provide shelter from the current and predators for many small trout. Usually you won't know one is there until you spook one or catch one. Look for rocks that provide easy access to drifting food.
Sometimes stream vegetation growth takes over the edges of these undercut banks and provides a natural shelf.
Overhanging Branches
Overhanging branches could be a few feet above the water or just a few inches. In the heat of the summer, trout find shade under these whether it's to blend in with the shadows or escape the sun.
Ants, beetles, and other terrestrial insects will sometimes fall off the branches to the awaiting trout below. Overall, there is an advantage to feeding trout that sit below low hanging branches.
Undercut Banks
Along river bends and deep straights, undercut banks provide safe shelter to trout and access to plenty of food. I imagine trout love these because they can remain unseen in these banks and not have to move for food. Banks that have been eroded from flooding act as a natural funnel that push drifting insects straight into the bend and into these eroded banks. Therefore, trout rarely have to move for food. Casting to these trout can be rather simple when nymphing and swinging streams, but for dry flies you'll want to cast just outside the undercut bank for trout to dart out and take them.
Deep Pockets
Deep pockets behind large rocks or following a riffle provide comfortable spots for feeding trout. Think about it, the deeper the pocket the easier it is to escape predators and get comfortable oxygen or temperature levels. These troughs and pockets help funnel drifting insects into these deep runs and make it incredibly simple for trout to get an easy meal.
Thick Aquatic Vegetation
Adult trout need cover to conceal themselves from predators and aquatic plants do just that. Aquatic vegetation provides the perfect habitat for small wild trout and a huge buffet of aquatic insects.
Shadows
Trout use shadows to conceal themselves because otherwise they would be 'naked' in the direct sunlight. They tend to hang along the edges of the shadows like they would with seams along soft and fast water.