ESRI's Geoportal is an open-source metadata catalog system capable of providing an OGC CSW Service. Due to its good documentation and relatively easy workflow for handling custom metadata profiles, USGIN has adopted Geoportal as its primary catalog system.
A Map Service is the standard web service provided by ArcGIS Server. These services can be produced by simply "publishing" an ArcMap document (.mxd) to an active ArcGIS Server. Benefits of using ESRI Map Services include utilization of the rich cartographic capabilities of the ArcGIS Desktop environment, relatively simply production, native integration with other ArcGIS Desktop products, and a rich and well-documented set of APIs for providing services in web applications.
Although this services are quite easy to produce, there are a number of important things to consider during the production process. More information about building ESRI Map Services can be found on this website as well as through ESRI's help documents.