A single language can comprise several different dialects, which are often dictated by geography. Consider the word aunt. In the Northeast of the United States, the word is often pronounced "ɔnt"; in the South, however, one often hears it pronounced "aint"; in the Midwest, it is pronounced "ant".
So it was with Ancient Greek. Unlike American English, however, differences in Greek dialects led to differences not only in pronunciation, but in spelling, as well. Thus, in the Attic dialect the word for "sea" is θάλαττα, with a "t" sound. In the Ionic and Doric dialects, however, the word is spelled θάλασσα, with an "s" sound. Sometimes vowel sounds shift between dialects. For example, the Attic word μήτηρ ("mother") in Doric becomes μάτηρ. Ancient Greek has a number of dialects, chief among them being Attic, Ionic, Doric and Aeolic. Due to its prevalence in ancient Greek literature, Attic Greek is considered the "standard" form of the language, and is the dialect studied in most Greek language textbooks.
Just as in English, the main feature distinguishing Attic from other contemporary dialects is the vowels. Unless it came after an ι, ε, or ρ, what was originally an ἄλφα usually shifted to an ἦτα (whence Attic "μήτηρ"). Another feature distinguishing Attic was a process called "contraction." What this means is that when two vowels encounter each other during morphological change they often contract to form one vowel or diphthong in order to be pronounced more easily. The student needn't worry about this much to begin with, but it will become relevant later on. For now, note that Attic was just one of many dialects in a diverse region that would eventually become Greece, and that these features distinguish the way the Athenians spoke.