illustration (attribution, if any possible, is at the end of the article)
The five 'aggregates' (which means they're composed of 'sub'-phenomena that have some similitudes enough for them to be put in the same 'category' of phenomena) are:
• mental formations, and
• discriminative consciousness.
Sensations refer to the mental 'contact' with a stimulus. It is easily thought of as objective, but at the same time it's already a mental subjective phenomena: you don't see or hear or taste without the signal being mentally interpreted. You never see an apple: you see some colours that your mind interprets as the image of an apple… You never taste an apple: you taste some chemical components that your mind interprets as the taste of an apple (and many 'chemical' desserts use flavouring and could taste of apple without any apple getting close). How do you know two people even see the exact same colours, hear the exact same sounds (gplus.wallez.name/eKjeajLuZwp)?
Perception is a filtration process. The mind ignores a lot of signals and let some of the signals arise as the 'foreground': this is a mental process. For example, your ear is perfectly capable of capturing the sound of your heart beat, but your mind filters it out as 'irrelevant' (most of the time) and you simply don't hear it although it is audible.
Feelings refer to the impermanent, context-dependent and not just object-dependent, mental appropriation of a sensation as 'pleasant' and 'unpleasant'. Again, this is a mental phenomenon, the easiest way to see this is obviously that in some cases, we're talking about matters of 'taste' or education: some music might be 'unpleasant' and 'noise' to someone without the appropriate musical education, while it will be 'pleasant' and 'refined' to someone else.
Perceptions is the mental pattern-matching which associates the taste or sight of an apple to what "an apple" looks like, or what "an apple" tastes like. It is the association of a particular stimulus to a category: the transition from this particular fruit to the 'family' it belongs to.
Mental formations are the inferences made based on the categories, e.g. "apples are healthy vegetables" (poisonous mushrooms are not).
This will include thought habits, prejudices, wrong views, right views, scientific models and possibly also the knowledge of their limitations, etc. This will include conventions, vocabulary, categories, concepts, language.
As such, it is also where tendencies are perpetuated, clung to. Mental fabrications 'store' and 'create' and 'unfold' karma: it is where volitions, or choices, are appropriated as lessons (rightly or wrongly) which will 'inform' the next choices.
Discriminative consciousness is the consciousness we most easily tap into, the one we 'hear' in our heads, the one making judgements and running a commentary on them…
A classic mistake
One should not imagine the five aggregates as a tower: one on top of each other. As I did lightly hint to in the descriptions above, there is no real ordering. These aggregates are co-dependently arising: they exist in relation to each other.
For example, pleasant/unpleasant is both dependent on sensations (e.g. not only the pitch of the notes, but also simply their loudness), but it also depends on mental fabrications (e.g. the musical education). No amount of musical education will compensate for the unpleasantness of "too loud"… but it might definitely help hearing interesting music when others hear cacophony.
For example, consciousness does modify what sensations you filter as 'foreground' based on what you pay attention to: most of the time, you're probably not having noticeable sensations of your breathing, although you can easily experience them and although sometimes you will notice them without making a choice to do so —typically when you'll have difficulty breathing: so the consciousness can affect the sensations just like the sensations can affect the consciousness.
What's the point of the five aggregates?
The presentation of the aggregates is to help dissolve the illusion of the self.
It is helpful to remember they're aggregates: you don't have one sensation at a time, you can see many stimulus at once… you can e.g. focus intently on a tiny object but still see a blinking light in a corner. One will be focused sensation, the other will be more 'embracing' but you'll still 'see' both — while also ignoring some 'fixed' part of the background that your mind judges un-important right now (which doesn't mean the mind would always do so: appreciating a painting goes via looking at a fixed image…).
Typically, the 'self' is usually 'built' on appropriating one or several of the aggregates as 'me': at the sensation level, it is 'my' experience… At the mental fabrication level, it is 'my' history… At the consciousness level, it is 'my' voice… The aggregates help you see that this is a delusion in the sense that you don't control at all what's yours, you cannot isolate it from the rest, it's just a mis-labelling. What you call 'you' is a bundle of tendencies co-dependently arising and influencing one another, without any one of them independent or permanent or isolated.
A 'chariot' exists on the basis of the aggregation of parts… the concept of 'self', of 'me', similarly exists when the five aggregates are grasped as if constitutive of a larger phenomena.
This is not to deny that a phenomenon might emerge out of other phenomena. This is merely to keep in mind that changing a wheel of the chariot will not make it less of a chariot, no more than adding or offloading some baggage. This being said, if you're looking for the essence of the chariot by dismantling it, you will not find any… If you assume it will last forever, you'll be disappointed… If you think it is indestructible, the reality check might not be to your liking…
This is not to deny the arising of an idea of 'self'. This is to help you see the 'self' as it is, impermanent, without intrinsic core, without solidity or independence.
[This doesn't mean there's no choice… but it does accept that choice is itself bound within limits. You cannot choose to see what's green as 'red'. You can change the label if you like, but not the sensation. You can build, or drop, mental fabrications (hopefully, you'll build wholesome ones and drop ignorant ones, not the other way round) but you cannot talk appropriately about what you don't know, no matter how much you want to. Choice is limited, but this doesn't mean there's no choice at all.]
Clinging causes future suffering (the second Noble Truth): the five aggregates are the substrata for clinging and thus contribute to the causal origin of dukkha (unsatisfactoriness, 'suffering').
PS: I did simplify, I know so. I hope this posts helps the beginners and the curious. Maybe as a reminder it might also help others, maybe by raising a question (even if the answer is that this post over-simplified things, the enquiry following the question might be worth it).