LZ77 and LZ78
Lempel-Ziv compression looks for repeated strings and replaces them with references or dictionary entries.
What it is
LZ compression is dictionary-style compression. Instead of only looking at single symbols, it looks for longer repeated patterns: words, byte sequences, image data, or other chunks that have appeared before.
Origin
The core LZ77 and LZ78 ideas were published by Abraham Lempel and Jacob Ziv in the late 1970s. Their work influenced a large family of practical compressors and file formats.
LZ77
LZ77 looks backward into recently seen data. If a sequence appears again, the compressor can store a pointer to the earlier copy instead of writing the sequence again.
LZ78
LZ78 builds a dictionary of sequences as it reads the data. Later appearances can be represented using dictionary references.
Why it matters
LZ ideas helped shape many everyday compression systems. ZIP-style compression, PNG image compression, GIF-era dictionary compression, and many general-purpose compression tools are part of the same broad family tree.