I’m assuming you’re looking for public transport real-time feeds.
If you’re after JSON encoded responses, you can check out the Trip Planner API. You should be able to get example requests using the Explore API function on the Open Data Hub.
Some of the other real time APIs are provided in the GTFS-realtime standard, which uses Protocol Buffers to encode the data. There are other forum threads which go into a bit more detail on how you can use these feeds.