Data Access Layer (DAL): standardized access mechanisms to distributed data objects
Simple Cone Search
Simple Image Access
Simple Line Access
Simple Spectral Access
Table Access Protocol

Uniform Content Descriptors (UCD): common language for metadata definitions for the VO
UCD1+ Controlled Vocabulary

VO Query Language (VQL, ADQL): standard query language (beyond the limitations of SQL)
IVOA Astronomical Data Query Language
IVOA SkyNode Interface

VOTable: XML markup standard for astro tables
VOTable Format Specification