Time is what you are wasting when making other plans. Time travels at an approximate rate of one second per second, for observers in an inertial frame.
Einstein's theory of relativity unites space and time (where time is a dimension) into a kind of elastic "sheet" that compresses in the direction of motion and forms dents in the presence of gravity. It also holds that space and time itself actually change to maintain the perceived velocity of light as a constant. Light (or a photon) has a constant velocity of c - it will change its position by about 300,000 kilometers in every second of time for all observers, no matter what their inertial frame of reference.
There are two fundamental views of time — the 'dynamic' and the 'static' view. The dynamic view holds that time is somehow 'flowing' — that only the present now exists, the past doesn't exist anymore and the future doesn't exist yet. Many people have this intuition, yet it is notoriously difficult to explain it in a coherent way. "Time flows" — what does that mean? How fast does it flow? At one second per a second? At one day per a day? Central to this view of time is the concept of 'now'; yet now is inherently elusive — as soon as you mention it, it is not 'now' anymore, but instead then.
The static view of time, by contrast, sees time as much like space. Past, present, and future all exist, at their respective times. The proposition "X exists at time T" is equally true whether T is before 'now' or after 'now' or right 'now'. This is a view of the world which gets rid of any special status for 'now'; it is simply an empty indexical, a product of language rather than some deep aspect of reality.
J. M. E. McTaggart, in his book The Unreality of Time, called the dynamic view A-series and the static view B-series. He argues that both views of time are inadequate, which he sees as proof that time itself is ultimately unreal and an illusion.
Modern physics generally assumes the static view of time. The dynamic view of time is assumed to be a product of human psychology rather than a fundamental aspect of reality. In general relativity and quantum cosmology, time and space are no longer viewed as fundamentally distinct; that time and space are incommensurable is an essential part of the dynamic view.
 In theology
These different views of time can also be seen expressed in different positions in Christian theology. The viewpoint of classical theism supposes that God exists outside of time, which is to say eternally. This inevitably implies a static view of time, since every moment to God is seen as equally present. By contrast, process theology supposes that God exists in time, and assumes fundamentally a dynamic view of time. This view of God ends up being more limited — even God doesn't know what will happen in the future, because the future hasn't happened yet. Process theologians see the future not yet existing as being essential for the possibility of free will; adherents of classical theism claim that free will is still possible in their view, although their explanations of how this comes about — most commonly, middle knowledge — are very hard to comprehend.
 Arrow of time
The dynamic view of time includes the concept of time having an arrow — time flows, and it flows only in one direction, forward — from past to the future; not backward, from the future to the past. Adherents of a static view do not believe this apparent directionality is a fundamental property of time itself; it is an emergent phenomenon due to effective time asymmetries in the laws of physics, such as the second law of thermodynamics.
 Extent of time
The different possible views about the extent of time:
- time is finite having a beginning and an end — this corresponds e.g. to a cosmology of Big Bang followed by a Big Crunch
- time is infinite having a beginning but no end — this corresponds to the Big Bang followed by an endless expansion of the universe. It is also the traditional view of Christian theology (time began when God created the world, but time will never end due to everlastingness of the heavenly reward / hellish punishment.)
- time is infinite having no beginning and no end — this corresponds to steady state cosmology. It was the traditional view in Greek and Roman thought, prior to the triumph of Christianity
- time is infinite having no beginning but it does have an end — it appears that no one has ever seriously subscribed to this view, but it is theoretically possible
- time is finite but having no beginning and end — this would require circular time, in which the future is followed by the past and the past is preceded by the future (as a circle is finite yet unbounded)
Friedrich Nietzsche's doctrine of eternal recurrence could be seen as corresponding to the last possibility, in which the history of the entire universe (and hence one's own life within it) is repeated endlessly, exactly the same every time.
One frequently hears it stated that non-Western cultures (e.g. indigenous cultures; Asian cultures such as Hinduism or Buddhism) have a 'cyclic' rather than 'linear' view of time. However, although they see history as composed of cycles of progress and regression, the successive cycles are not identical to each other but distinct. Things repeat, similarly each time, but not exactly the same each time. This means their view is really that of "time is infinite having no beginning and no end", rather than "time is finite but having no beginning and no end".