One is a half circle, one is a full circle, they both seem to nasalize the preceding vowel, but what is the difference in terms of IPA, or another way to describe it?
The one like a circle is called sunna (anusvara in Sanskrit) and denotes a nasal consonant of the same place of articulation as the following consonant:
రంగు (raṅgu) /raŋgu/ “color”
పండు (paṇḍu) /paɳɖu/ “fruit”
The one like a semicircle is arasunna (chandrabindu in Sanskrit) and it is not used anymore in the modern Telugu. In the old Telugu it denoted “historical nasal consonant” which disappeared in the process of the language development, but which is restored in some positions:
తమ్ముఁడు (tammũḍu) “younger brother” > తమ్మునికి (tammu-ni-ki) “to a younger brother”
Telugu has no nasalized vowels, writing arasunna is just a spelling convention, it is not pronounced.