AVI (Audio Video Interleave) is a multimedia container format introduced by Microsoft in 1992 as part of Windows Video for Windows. AVI containers interleave audio and video data — hence the name — and can contain various video codecs (DivX, Xvid, H.264, uncompressed). AVI was the dominant video format in the 1990s-2000s.
AVI has significant limitations compared to modern containers: it doesn't support streaming (must download entirely before playing), has limited subtitle support, doesn't support modern metadata standards, and the container overhead makes files slightly larger than equivalent MP4. Files recorded with older codecs (DivX, Xvid) can be large and have compatibility issues with modern devices.
If you have old AVI files, convert them to MP4 using FileCurve's video converter for better compatibility and smaller file sizes. AVI files that contain H.264 video can be quickly remuxed to MP4 without re-encoding (no quality loss). AVI files with older codecs need full re-encoding, which takes longer but produces a universally compatible MP4.