There are many varieties of Slovak. These may be divided in four basic groups:
The fourth group of dialects is often not considered a separate group, but a subgroup of Central and Western Slovak dialects (see e.g. Štolc, 1968), but it is currently undergoing changes due to contact with surrounding languages (Serbian, Romanian and Hungarian) and long-time geographical separation from Slovakia (see the studies in Zborník Spolku vojvodinských slovakistov , e.g. Dudok, 1993).
For an external map of the three groups in Slovakia see here.
The dialect groups differ mostly in phonology, vocabulary and inflection. Syntactic differences are minor. Central Slovak forms the basis of the present-day standard language. Not all dialects are fully mutually intelligible. It may be impossible for an inhabitant of the Slovak capital Bratislava (in western Slovakia) to understand a person from eastern Slovakia. Also, it is primarily western dialects that can be considered fully mutually intelligible with the Czech language.
The dialects are fragmented geographically, separated by numerous mountain ranges. The first three groups already existed in the 10th century. All of them are spoken by the Slovaks outside Slovakia (USA, Canada, Croatian Slavonia, Bulgaria and elsewhere) and Central and Western dialects form the basis of the Lowland dialects (see above).
The western dialects contain features common with the Moravian dialects in the Czech Republic, the southern central dialects contain a few features common with South Slavic languages, and the eastern dialects a few features common with Polish and the East Slavonic languages (cf. Štolc, 1994). Lowland dialects share some words and areal features with the languages surrounding them (Serbian, Hungarian and Romanian).