IT Governance ensures that
- IT delivery expectations are fulfilled
- IT resources deployment is continuously planned, targeted and optimized
- IT performance is measurable
- and that the risks are minimized
We may consider defining an IT Governance team which would care of various domains such as Project Portfolio Management, Security Management, Project Management, COBIT, SOX, Risk Management ,Service Management, ISO 9000, etc..
Enterprise Architecture Governance encompasses leadership, implementation and controlling of Business Architectures, IT Architectures (Information, Application, Technology architectures) and Solution Architectures including organizational structures (organizational units as well as processes and roles) to ensure that architecture sustains and extends the business strategy and objectives. When we implement an EA Governance, we may consider defining an Architecture Review Board which decides which IS is best suited for the needs of the enterprise, it decides when a change in the architecture is needed, and prioritizes initiatives. More detailed explanation on this other post.
To summarize Enterprise Architecture is one of the IT Governance pillars and Enterprise Architecture with its associated Governance is s subdomain.