These are respectively two different forms of practice known as samatha (tranquility), and vipassana (insight). Samatha takes one subject, usually from the six listed under mindfulness of the body (Satipatthana Sutta, DN 22 or MN 10):
“The topic of the body in and of itself covers six exercises: (1) the first four steps of breath meditation; (2) the practice of discerning whatever posture the body is in; (3) the practice of making yourself alert in all your physical activities; (4) the analysis of the body into 31 parts; (5) the analysis of the body into the four physical properties in every posture; and (6) the practice of visualizing a corpse in nine stages of decomposition, and reflecting that your body will unavoidably meet with the same fate. MN 117 lists all six of these exercises under the term, “mindfulness immersed in the body” (kayagatasati).”—-“Right Mindfulness”, Ven. Thanissaro.
Insight meditation has changing subjects according to the experiences that are happening at the time, and strives to apply dhamma principles such as impermanence to events in daily life.
“It might be assumed that we are always aware of the present, but this is a mirage. Only seldom do we become aware of the present in the precise way required by the practice of mindfulness. In ordinary consciousness the mind begins a cognitive process with some impression given in the present, but it does not stay with it. Instead it uses the immediate impression as a springboard for building blocks of mental constructs which remove it from the sheer facticity of the datum. The cognitive process is generally interpretative. The mind perceives its object free from conceptualization only briefly. Then, immediately after grasping the initial impression, it launches on a course of ideation by which it seeks to interpret the object to itself, to make it intelligible in terms of its own categories and assumptions. To bring this about the mind posits concepts, joins the concepts into constructs — sets of mutually corroborative concepts — then weaves the constructs together into complex interpretative schemes. In the end the original direct experience has been overrun by ideation and the presented object appears only dimly through dense layers of ideas and views, like the moon through a layer of clouds.
The Buddha calls this process of mental construction papañca, "elaboration," "embellishment," or "conceptual proliferation." The elaborations block out the presentational immediacy of phenomena; they let us know the object only "at a distance," not as it really is. But the elaborations do not only screen cognition; they also serve as a basis for projections. The deluded mind, cloaked in ignorance, projects its own internal constructs outwardly, ascribing them to the object as if they really belonged to it. As a result, what we know as the final object of cognition, what we use as the basis for our values, plans, and actions, is a patchwork product, not the original article. To be sure, the product is not wholly illusion, not sheer fantasy. It takes whatis given in immediate experience as its groundwork and raw material, but along with this it includes something else: the embellishments fabricated by the mind.
The springs for this process of fabrication, hidden from view, are the latent defilements. The defilements create the embellishments, project them outwardly, and use them as hooks for coming to the surface, where they cause further distortion. To correct the erroneous notions is the task of wisdom, but for wisdom to discharge its work effectively, it needs direct access to the object as it is in itself, uncluttered by the conceptual elaborations. The task of right mindfulness is to clear up the cognitive field. Mindfulness brings to light experience in its pure immediacy.”—-“The Noble Eightfold Path,” Bikkhu Bodhi.