A tea infuser is part of the tea brewing process, it holds the loose leaf tea in a confined area while your tea steeps, then it is used to strain the tea leaves out of the water, leaving you with your steep tea. A tea strainer is used towards the end of the tea brewing process. The tea leaves are added to the teapot for brewing tea, once done a strainer is used when pouring the tea into a cup, which traps the tea leaves but allows the steeped water to go through. Just keep in mind that many will use them interchangeably.