To be classed as "true" high definition, we encode in at least 1280x720 resolution or 720p. We use h.264 with a bitrate of 3.2Mbps (going up to 5Mbps for best quality, should your connection allow) and 192kbps audio. This means that streaming programmes in HD quality will put demands on your network connection.
Standard Definition is any resolution which streams at lower than 720p. SD programmes usually require less bandwidth to stream and download and won't put as much demand on your network connection as HD programmes.