This is (arguably) the most important work written by Deleuze for a reason that seems to me is often obscured or merely forgotten: it is (maybe) the only work that seeks to lay the foundation for a systematic treatment of 'difference' and by ex-tension (or in-tension) 'repetition'. It does not seek to derive 'difference' and 'repetition' (simply) from identity and the in-dividual. It seeks to think of 'difference' and 'repetition' in themselves. And this is what is important here: thinking (and not some petty play of figures and words in the frontal attacks or soul mating with particular thinkers) in its rhizomatic form rather than its arborescent one.
What is therefore central in this work is 'idea', and (therefore) 'perception'. In simple terms, Deleuze has managed to provide us with some foundational links with the philosophies of mind, language and time (and moreover besides). He has given to the philosophy of difference a central and unifying role (across such and other disciplines) to play.
In this sense 'difference' and 'repetition' are not only (simply) linked between them (in the sense that one leads to the other), but also linked with other important notions usually discussed and developed in other (philosophical) disciplines. Let me provide some brief indications.
Chapter 1 is concerned with 'difference', not as mere 'diversity', 'otherness' or 'negation', bur rather as 'general' or 'specific' difference, where the latter refers to the moment when difference is reconciled with the concept in general. In this manner, Deleuze sees 'difference' as a concept of reflection in relation to 'representation' that involves 'movement'. He further discusses the notion of 'eternal return' and questions the adoption of a 'meta-viewpoint' for thinking about 'difference' and 'repetition' - the latter being the relation between originals and simulacra.
In chapter 2, Deleuze lays out the relation between (the dualities) 'repetition' and 'sensing', 'habit', and 'difference', under the guise that "difference inhabits repetition", in that it "lies between two repetitions" (p.76). He also makes the distinction between 'natural' and 'artificial' signs, hence the distinction between two types of 'difference', one being the expression of the other. In parallel, he distinguishes 'active' from 'passive' synthesis (relative to time) in that "the activity of thought applies to a receptive being, to a passive subject" (p.86). Finally drawing on Bergson, he distinguishes the 'real' centre from where emanates a series of 'perception-images' from a 'virtual' centre from where emanates a series of 'memory-images'.
Chapter 3 is for Deleuze the most important (sic) because the thinking of 'difference' and 'repetition' is based on a dogmatic image of thought characterised by eight postulates, each with a dual form, the artificial and the natural.
In Chapter 4, this duality underlies the development of the notion of 'idea' in that it is problematic, hence dialectical, an "n-dimensional, continuous, defined multiplicity" (p.182) in a 'perplication' as the distinctive and coexistent state of ideas. Each 'idea' is thus linked with 'difference' and 'representation' in that "the representation of difference refers to the identity of the concept as its principle" (p.178). In this manner he makes the claim for the superiority of problematic-questioning approach over the (traditional) hypothetico-apodictic approach because questions are imperatives.
Chapter 5 starts with the claim that "difference is not diversity. Diversity is given, but difference is that by which the given is given, that by which the given is given as diverse" (p.222). Difference is therefore (a given) 'intensity' expressed as 'extensity'. There is 'depth' that unites intensity and extensity. Therefore, 'depth' is the intensity of being from where emerge at once extensity and the qualities of being. In this manner Deleuze accepts a dual condition of difference: one natural and one artificial.
In the concluding chapter Deleuze argues that 'representation' is a site of transcendental illusion which comes in four interrelated forms relative to 'thought', 'sensibility', 'idea' and 'being'. Hence the problematic of 'grounding' representation and his argument (or Idea) for 'groundlessness', and the justification of the use of (systems of) 'simulacra' as sites for the actualisation of ideas. Hence that of 'difference' and 'repetition' where the former is not only located between the levels and degrees of the latter, but also has two faces, namely, habit and memory.
Overall, despite the difficulty of the text itself as it takes for granted knowledge of the philosophies of some other thinkers (e.g. Bergson), it is a central text in the philosophy of difference and for just this reason, a text one must have read!