The major division is between the Eastern and Western dialects. The most distinctive feature of Western Armenian is that it has undergone several phonetic mergers; these may be due to proximity to Arabic and Turkish-speaking communities.
For example, Eastern Armenian speakers pronounce (թ) as an aspirated "t" as in "tiger", (դ) like the "d" in "develop", and (տ) as an unaspirated voiceless stop, sounding somewhere between the two as in "stop." Western Armenian has simplified the stop system into a simple division between voiced stops and voiceless aspirate ones; the first series corresponds to the unaspirated voiceless series of Eastern Armenian, and the second corresponds to the Eastern voiced and aspirated voiceless series. Thus, the Western dialect pronounces both (թ) and (դ) as an aspirated "t" as in "tiger," and the (տ) letter is pronounced like the letter "d" as in "develop."
Armenian can be subdivided in two major dialectal blocks and those blocks into individual dialects, though many of the Western Armenian dialects have died due to the effects of the Armenian Genocide. In addition, neither dialect is completely homogeneous: any dialect can be subdivided into several subdialects. While Western and Eastern Armenian are often described as different dialects of the same language, some subdialects are not readily mutually intelligible. It is true, however, that a fluent speaker of two greatly varying subdialects who are exposed to the other dialect over even a short period of time will be able to understand the other with relative ease.
English - Eastern Armenian
English - Western Armenian