STIX (Structured Threat Information eXpression) is a JSON format for threat intelligence.
TAXII (Trusted Automated eXchange of Indicator Information) is the transport protocol for sharing STIX data.
STIX + TAXII = the standard for machine-readable TI sharing. ISACs use TAXII servers to distribute sector-specific TI. SIEM integrations consume STIX bundles to automatically create detection rules. More durable than manual IoC sharing.