There are five different training intensities for rowing and each has its own specific physiological benefit. Doing one type instead of another does not help your performance in the correct way.
This is the lowest intensity exercise. This is to train and improve your oxygen utilisation by the muscles. Pulse is usually between 130-150 for most people. You should easily be able to carry on a conversation whilst at this intensity. The duration of the session is the longest, usually 45-90 minutes. The session should be as continuous as possible - breaks should be just long enough to grab a swig of water. If your pulse rate drops more than about 15-20 bpm during your break it has the same effect as starting the session all over again.
This is the next training level. This also improves your oxygen utilisation by the muscles, but it builds on the UT2 and widens your aerobic fitness base. Pulse at around 145-165. Intermittant conversation is possible. Here the session is slightly shorter (25-45 minutes) and can be broken up into multiple pieces of work.
Here the aim is to increase the amount of power you produce at your anaerobic threshold. Once you go beyond your threshold there is a build up of lactic acid. It is important to be able to produce the maximum power possible at that level before going anaerobic. Pulse 170-180. Hard work, feeling a burning in the limbs which is sustainable for extended periods. This is physiologically the most taxing session and must not be done too often, particularly if not well trained.
This session is aimed at improving the transport of oxygen by the heart to the muscles. Pulse 175 and upward. This is achieved by a series of hard shortish pieces, which are quite hurtful.
These sessions affect the ability of your blood to buffer the lactic acid. The aim is to enable you to work at absolute maximum power for extended periods of time. Pulse rate is at maximum. These pieces are at full power.