What is concrete?

In its simplest form, concrete is a mixture of paste and aggregates (sand & rock). The paste, a combination of cement and water, coats the surface of the fine (sand) and coarse aggregates (rocks) and binds them together into a rock-like mass known as concrete.

Source: NRMCA

Isn't cement and concrete the same thing?

Contrary to popular belief, concrete and cement are not the same thing. Just like flour in a cake mix, cement is an "ingredient" in concrete. Cement acts as the binding agent when mixed with water and aggregates from concrete.

Source: Concrete Network

Why does concrete crack?

The majority of concrete cracks usually occur due to improper design and construction practices, such as:
a. Ommission of isolation and contraction joints and improper jointing practices.
b. Improper sub-grade preparation.
c. The use of high slump concrete or excessive addition of water on the job.
d. Improper finishing
e. Inadequate or no curing.

Source: NRMCA Concrete in Practice #4
For more information on concrete cracks see the following: CIP5, CIP6, CIP7

What special attention needs to be paid when pouring concrete in hot weather?

High temperatures cause increased water demand which, in turn, will raise the water-cement ration and result in lower potential strength. Higher temperatures cause a rapid rate of evaporation of moisture from the surface of the concrete and accelerated setting time.

Source: NRMCA Concrete in Practice #12

What special attention needs to be paid when pouring concrete in cold weather?

Low concrete temperature has a major effect on the rate of cement hydration, which results in slower setting and rate of strength gain.

Source: NRMCA Concrete in Practice #27