What is the difference between Yogacara Buddhism and Idealism?

Yogacara is a form of idealism. Classical subjective idealism developed by George Berkeley says that everything that we think to be external objects are really just purely internal representations, and Yogacara says the same thing.

However, Yogacara is a lot more than just saying that. Yogacara also has detailed descriptions for how this process of mental projection occurs within the mind, how it is dictated by karma, and how to meditate to gain insight into how all things are mind created.

While idealism (1) gives some substantial existence to ideas and (2) separates neatly inner and outer worlds, in Buddhism (mahayana) (1) ideas and emotions (inner world) are as much unsubstantial as "matter" (outer world) is and (2) the concepts of inner and outer reality still belong to a dualistic, relative view, hence are not elucidating what the true nature of reality is (true nature is non-dual). Mahayana Buddhism practices sometime "prioritize" mind over matter for the only pragmatical reason that mind can usually be controlled directly, broadly and more efficiently, but at one point of training, mind (thinking mind) should be overcome too in order to experience the true nature (enlightment). Mahayana don't idealizes mind or ideas thus, IMO, it is not a form of idealism...

I would like to say that it depends on the Yogacara school (and how broadly are you willing to define idealism). The original school does NOT say that everything is consciousness, it only say that everything cannot be INDEPENDENT FROM consciousness. It's important to understand this fundamental difference as Yogacara defines mind differently from Western philosophy. In western idealism, the mind, self, and concepts are not fundamentally separate. However, this is duly separated in Yogacara philosophy. According to the Faxiang school, the physical reality conventionally EXIST, they are NOT pure conceptual constructs; however they are only real for you and not others. The conceptually constructed realm (遍计所执 parikalpita-svabhāva) is only the first of the 3 self natures and is the only one that is completely not real. The realm of causal dependency (依他起性 paratantra-svabhāva) is conventionally REAL, it is basically reality as perceived from a tainted lense. So to use an analogy, you see a cup for example, the concept of the cup is the first parikalpita-svabhāva, however, there is still a real cup there, if it was purely conceptual, it would be like a dream, but its not. This real cup is however tainted, but the experience of the cup is real and is the second paratantra-svabhāva. However, this cup is only real to the observer. Outisde of the observer there is no cup. Once a person gets rid of his defilements and see the cup's true nature as empty, he then sees the tathata which is the only thing thats ultimately real and not as a cup. Idealism assumes that the concept of the cup is produced by the mind and is hence just conceptual. Yogacara accepts that there is a concept of a cup and a real cup divorced from concepts, hence it is NOT idealism. However this real cup is not really a cup.

It's important also to note that the mind spoken of in Western idealism is only restricted to what Yogacara calls the sixth consciousness. Using this definition, Yogaracara is definitely not idealism, as it does not suggest that the sixth consciousness produces external reality and ultimately exist. The sixth consciousness is itself dependent on conditions stored in seeds based on your previous actions and mental habits (as well as on others). Yogacara also defines "real" differently from western philosophy. In the west what is real is usually associated with what fundamentally exist. Yet in Yogacara nothing substantially exist because everything is dependent and empty, but they are still real because you experience it. Contrast to Kant, only what you experience is real (because its a priori). An external metaphysical thing in itself is not (metaphysical, objective existence is denied as real altogether because of the idea of emptiness). External reality is real in Yogacara, but does not objectively exist and early writers even mention things like rupa dharma as conventionally real and only exist because of your actions (karma), but its not substantially objective, as other people cannot experience what you experience.

Here's one additional point. The other very astute answers have dealt with possible philosophical distinctions between these two. The only point lacking as a difference between these two in the other excellent answers I see is that Yogacara Buddhism is not simply a philosophical take on reality or experience. So, in elaborating the difference between them, it must also be pointed out that Yogacara Buddhism includes so much more than just philosophical view or distinctions.

The name itself "Yogacara" means (or indicates) something like "one who masters the practice of yoga." So Yogacara Buddhism includes a system of training on which the whole thing is predicated. Honestly, I'm not a trained western philosopher, so I don't know if the exponents of Idealism et al included progressive systems of mind training designed to transform one's experience of perception, interpretation, etc, but I think it could be successfully argued that this is the sole and only point of Yogacara (and any Buddhist system).

So while trying to fit a Buddhist system into a frame like idealistic or not, ontological or not, phenomenological or not, may be an interesting intellectual exercise from the perspective of scholars trying to understand the tradition from the outside or even practitioners on the inside, don't miss the main point which is that Yogacara Buddhism (and any Buddhism) is soteriological or aimed at liberation from suffering and ignorance first and foremost. Therefore, the positions, statements, etc, are not made to color within the lines of a particular perspective (commitment to idealism or not, in this example) but instead are made for their efficacy in producing liberation for a practitioner.

I suggest this is why you cannot group the whole of every significant writer on the topic of Yogacara into a single uniform lot and easily conclude, "They all believed the same thing." Also why scholars studying the subject (as outlined in prior answers) can't come to consensus about the matter. This just isn't something Yogacara masters were really concerned with. Some writers lean more idealistic than others, and they differ in what aspects of the system they emphasize at different times. Even the same author can sometimes be found to write one text that seems more idealistic and another that seems less idealistic.

Just two more cents.