What are their common characteristics? I was reading about it on Wikipedia but didn't understand much since I have no background in linguistics. I would appreciate if someone could just name some basic common features (without too much rigor if possible) and give examples. Thank you!
If you look at the aspect system of Baltic and Slavonic languages, Baltic systems actually resemble the earlier stages of Slavonic systems (Comrie, 1976). In Lithuanian, adding a prefix to a verb root renders it Perfective, sometimes resulting also in some other semantic change. There is also a suffix -inè, albeit with limited productivity, which changes these prefixed verbs back into Imperfective. Latvian is similar, though without this suffix.
This in fact echoes the paths of change in language such as Russian. In the beginning there was a verb, say Russian pisat' ('write'), to which would be added a prefix to form perfective form, either without change in meaning, as in Russian na-pisat' ('write'), or with such a change, as in vy-pisat' ('write out'). Latvian is still at this stage. Later, Imperfective versions of those verbs which gained more-than-aspectual prefixes would appear, so the Russian word vy-pisyvat appeared. Lithuanian is going through this stage.
Source: Comrie, B. (1976). Aspect: An introduction to the study of verbal aspect and related problems. Cambridge University Press.