Both QGIS and GRASS GIS are standalone applications which can be used on their own. QGIS is using some of the functionality from GRASS GIS through QGIS GRASS Plugin and through QGIS Processing. Each of these can support some of the tools available in GRASS GIS or somehow modifies their usage. In this context, the functions or tools in GRASS GIS are called modules. Not all these modules are supported by QGIS, either because they simply were not added to the list or QGIS is not able to accommodate their interface.
Both QGIS GRASS Plugin and QGIS Processing support both GRASS GIS 6 and 7. QGIS is currently transitioning from version 6 to 7, so the different distributions of QGIS can include different versions. The plugin or Processing can be configured to use one or the other once it is installed. In OSGeo4W, GRASS GIS 7 is currently probably in an advanced installation.
Some of the GRASS GIS modules are not part of the standard GRASS GIS installation and their are available in GRASS GIS Addons repository. You can install them in GRASS GIS using GUI or
g.extension module. These cannot be used in QGIS because Processing nor the plugin (at least currently) support arbitrary modules, just the ones specified in QGIS. For example,
i.segment was an addon at one point but now it is included in GRASS GIS directly. Some addons stay just addons.
If you want full access to everything in GRASS GIS, use GRASS GIS directly. QGIS GRASS Plugin will help you transition back and forth. A lot of people are using this workflow.
This was a general answer which seems appropriate now. If you have specific issues with using QGIS and GRASS GIS together, I suggest you to search the existing questions first and if this doesn't help, ask another one.