In her introduction, Galia Hatav (see bibliography) briefly mentions that in recent semantic studies the term aktionsart has generally been reserved for the description of aspectual notions, such as events versus states, which are NOT morphologically marked in the language (ie by some sort of verbal inflection). In contrast, aspect is the term used to describe these morphological distinctions. In my reading and class discussions, I have found that these two terms are often misunderstood and confused, but there is actually a good reason for that.
While the differences between tense and aspect were discussed generally by Greek and Roman thinkers, in modern linguistic thought aspect was formally distinguished from tense by Russian linguists. The term “aspect” is a calque from the Russian vid meaning “view”. At first it was treated as part of the tense system, but began to be distinguished as its own formal category by the early 1900’s. Meanwhile, German grammarians were being influenced by the Russian work. Aspect was also described as the “circumstance of the action” leading to the German term Aktionsart. At this point, both Vid and Aktionsart seem to have been broad terms describing the general notion of “aspectuality” in distinction from tense.
As the study of “aspectuality” progressed, the Slavic linguists realized that it was indicated broadly in the external morphology of the verbal forms (perfect/imperfect) but also more exactly by some inner quality of the verb itself. Thus within “aspectuality” the aspect, marked externally, became distinguished from the aktionsart, the inner quality. However, German linguists questioned whether the narrow definition of aspect as a binary perfect/imperfect distinction could be applied to Germanic languages, or whether it was a feature of Slavic languages. Further, while the basic distinction of aspect and aktionsart became accepted within Slavic linguistics, much work remained to formally define the two categories.
The German linguists moved toward the position that notions of aspect can be expressed in different ways in different languages. While some early work tried to apply the Slavic notion of perfective/imperfective, aspectuality in Germanic languages seemed to be realized in many different ways and in many different categories such as terminative/aterminative, telic/atelic, bound/unbound, etc. Thus, the distinction of aspect from aktionsart is not strictly based on morphological criteria. This Western tradition seems to elevate semantics over morphology.
The Slavic linguists focused on morphologically marked pairs indicating aspect, but aktionsart was more difficult to define. While aspect was seen as the subjective view of the speaker, aktionsart was seen as an objective lexical quality of the verb. Further, aktionsart is not constrained by binary opposition. Under the opposition of perfective/imperfective are several categories such as ingressive, iterative, etc.
So what does this mean for you? Well, it is important to recognize that there are two major traditions of linguistics which treat aspect vs aktionsart slightly differently. To perhaps oversimplify, the Germans focused on the semantic nature of aktionsart, but had a difficult time formalizing aspect, while the Slavic linguists focused on morphologically marked aspect, but had a difficult time formalizing aktionsart. More confusing, older German works may still use Aktionsart in the more general sense of “aspectuality” in distinction from tense, rather then in distinction to “aspect”. Lastly, while it is intuitive that there is a distinction between aspect and tense, there is still much work being done on how aspect is realized in any given language and what the formal categories of aspect are. For instance, Hatav prefers a semantic truth-condition approach over the pragmatic approach which emphasizes the subjective view of the speaker. Aspect is also being investigated in relation to discourse function, etc. Don’t be too quick to assume you know what an author means when they begin to talk aspect.