There is no precise linguistic border between one dialect and another because there is nearly always a dialect transition zone of some size between pairs of geographically identified dialects, (except for dialects specific to an island). The main difference between both blocks is:
~ Western Catalan (Bloc o Branca del Català Occidental):
o Unstressed vowels: [a] [e] [i] [o] [u]. Distinctions between e and a and o and u.
o 1st person of Indicative's present desinence is -e or -o.
o Inchoative in -ix, -ixen, -isca
o Maintenance of medieval nasal plural in proparoxiton words: hòmens, jóvens
o Specific Vocabulary: espill, xiquet, granera, melic...
~ Eastern Catalan (Bloc o Branca del Català Oriental):
o Unstressed vowels [ə] [i] [u]. The unstressed vowels e and a becomes /ə/ and o and u becomes /u/.
o 1st person of Indicative's present desinence is -o, -i or ø.
o Inchoative in -eix, -eixen, -eixi.
o The -n- of medieval nasal plural falls in proparoxiton words: homes, joves.
o Specific Vocabulary: mirall, noi, escombra, llombrígol...
In addition, neither dialect is completely homogenous: any dialect can be subdivided into several sub dialects. Catalan can be subdivided in two major dialectal blocks and those blocks into individual dialects: