ATIS is for Automated (or Automatic, I can't recall which) Terminal Information Service, and it's a short endless loop tape recorded each hour by tower personnel that gives weather, runway in use, and NOTAM info. It's usually "cut" or recorded once an hour (when the new METAR comes out) can be updated sooner should conditions warrant. Each ATIS broadcast is given a letter, "Alpha", "Bravo", etc. so you can tell when it's been updated.
ASOS is for Automated Surface Observation System, and it's a system comprised of various sensors that actually take readings used in the hourly METAR report, stuff like wind direction/speed, cloud cover, temp/dewpoint, and baro pressure. The observations that ASOS creates are available via conventional computer access, but they are also broadcast on a VHF freq assigned to a specific ASOS, and some can even be accessed by phone. ASOS takes an observation every minute.
The main differences are that ASOS doesn't give you any runway in use info, or NOTAM info.
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