Web feature services come in two forms:
WFS -- the Open Geospatial Consortium standard format
Feature Access -- ESRI's proprietary feature service format
ArcMap and other ESRI applications are able to use both of these service formats. However, only Feature Access is supported without the Data Interoperability extension. Both of these feature service types allow features to be queried and edited like local vector datasets.
Adding a WFS to ArcMap
As mentioned, using a WFS in ArcMap requires that the Data Interoperability extension be installed. With this extension, a connection to a WFS can be made. To do so, go into the catalog window and find the Interoperability Connections item. Expand it, and double-click the Add Interoperability Connection tool. A window will pop up.
This window contains the settings for the interoperability connection. As we are wanting to add a WFS, first click on the ellipses to the right of the red Format dropdown. In the window that opens, scroll to near the bottom and select WFS, then click OK. Next, in the Dataset field, type in the URL of the desired WFS. If you are unsure of the URL, please contact the WFS administrator.
With the URL entered, click on the Parameters... button. This will open another window with many settings for the interoperability connection. All of the required settings will be highlighted in red. The first of these is the Feature Types field under Constraints. Click the ellipses to the right of the field, and the wizard will connect to the WFS to find all the different feature types in the service. From the results, check all the features types needed, then click OK. This should be the only required setting for a WFS, so click OK in the Parameters window. Lastly, click OK in the Interoperability Connection window.
The WFS connection should now be setup for use. Return to the catalog window, and the WFS should be listed under Interoperability Connections. Drag it into the ArcMap window, and it will be added to the map.
More about support for WFS and other open standards in ESRI applications can be found at the following web address:
Adding a Feature Access Service to ArcMap
As services with feature access are an ESRI format, they do not require an interoperability connection. Instead, all that is needed is a proper connection to the host server. To establish this connection, again visit the catalog window, but find the GIS Servers item. Expand it. Listed will be any existing server connections; if the correct connection is not present, add it by double-clicking the Add ArcGIS Server item. A wizard will open. Choose to the option to Use GIS services and click Next. In the Server URL field, enter the server's address, and add any user credentials needed for authentication in the User Name and Password fields, then click Finish.
The server connection should now appear under the GIS Servers item in the catalog window. Browse to the correct folder on the server and identify the correct service; Feature Access services have an icon with a little computer screen displaying a map, and a gray rectangle in the lower right corner. Drag the service into the ArcMap window and it will be added.