A block is a named, contiguous range of Unicode codepoints. Every assigned character lives inside exactly one block, and the entire codepoint space — from U+0000 through U+10FFFF — is carved up into these ranges by the Unicode Consortium. Block boundaries always align to multiples of 16, so the smallest possible block is 16 codepoints and most fall on rounder boundaries like 128, 256, or 4,096.
Blocks are not the same as scripts. A script is the writing system itself — Latin, Greek, Devanagari, Han — and a single script frequently spans many blocks: Latin script is housed in Basic Latin, Latin-1 Supplement, Latin Extended-A, Latin Extended-B, Latin Extended Additional, IPA Extensions, and others. Conversely, a single block can hold characters from several scripts, as in General Punctuation, which collects marks shared across writing systems. Blocks are an organizational convenience; scripts are a linguistic property.