ISCII (Indian Script Code for Information Interchange) is a coding scheme for representing various Indic scripts. Most of those scripts are rather similar in structure, but have different letter shapes. So ISCII tries to encode the logical structure of the Indic scripts, while using escape sequences to switch between language-specific letter shapes.
ISCII is a fixed-length 8-bit encoding. The lower 128 codepoints are plain ASCII, the upper 128 codepoints are ISCII-specific.
Unicode has largely preserved the ISCII encoding strategy, but has assigned each language a separate codepoint range. So now there is a series of 128-codepoints-long blocks for Indic scripts.