What is cosmology?
Cosmology is a study of the history of the universe from a perspective of physics. It addresses questions of the very large, such as:
What is the size of the universe?
How old is the universe?
Is the universe expanding?
What is the ultimate fate of the universe?
How did galaxies form?
To address these questions, cosmology uses the physics of the very small. The pressure that drives the expansion of the early universe can only be understood from a perspective of particle physics. By combining nuclear physics models with measurements of isotopic populations, one can ascertain the conditions of the early universe. Cosmology addresses not only the "what is the universe like?" question, but the "how and why" questions as well.
Two books are highly recommended: Stephen Hawkings A Brief History of Time (1988) and Steven Weinberg's The First Three Minutes (1977). Both are written at an appropriate level. There also exists a CD counterpart to Hawking's book by the same title.