The five aggregates (khandha, skandha) are the categories into which Buddhist analysis divides what is conventionally called a person: form, feeling, perception, formations and consciousness. The list is not offered as description for its own sake. It is an instrument: a way of looking for a self systematically and not finding one.
Rupa, form or materiality, the body and the physical, analysed in terms of the four great elements and what derives from them.
Vedana, feeling, the hedonic tone of any experience, pleasant, unpleasant or neutral. This is bare valence, not emotion; emotions are compounds and belong to the fourth aggregate.
Sanna, perception or apperception, the recognising and labelling of what is experienced, the function that identifies a colour as blue or a sound as a voice.
Sankhara, formations or volitional activities, much the widest and most heterogeneous of the five, covering intention, disposition, habit and the mental factors that shape action. It is the aggregate that carries karmic weight, since volition sits here, and the tradition's definition of karma as intention makes this the operative category.
Vinnana, consciousness, bare awareness arising in dependence on a sense faculty and its object, enumerated as six kinds according to the six senses.
The analysis is deployed argumentatively in the Anattalakkhana Sutta: each aggregate is examined, found to be impermanent and therefore unsatisfactory, and therefore not fit to be regarded as self. The first noble truth is stated in the same terms, in brief, the five aggregates subject to clinging are dukkha, so that the diagnosis and the analysis of the patient use one vocabulary.
Two points are regularly misunderstood. The list does not say that a person is nothing; it says that a person is a process with no unchanging core, and the aggregates themselves are categories of analysis rather than five things bolted together. And in Mahayana the Heart Sutra turns the analysis on the analysis, applying emptiness to the aggregates themselves, form is emptiness, emptiness is form, which is the characteristic Mahayana move of refusing to let the tool become a new absolute. Abhidharma literature elaborated the scheme into detailed enumerations of mental factors, and the Theravada and Sarvastivada lists differ in both number and arrangement.