here are two types of yeast available in the market: Active Dry Yeast and Instant Yeast.
Active dry yeast needs to be activated before use. To do this, mix together lukewarm water, sugar and yeast, stirring vigorously to ensure the yeast is fully dissolved. Cover and set aside in a warm place for 5 to 10 minutes. Once froth forms on top, your yeast is activated and ready to use. To get it right the first time, you need to remember two things. First, dip your fingers in the water to make sure it is lukewarm, around 105 – 110°F (37 – 40°C). If the water is too hot, the yeast will be killed, and if it is too cool it won’t activate. Next, remember to add the sugar, because it is the food on which the yeast feasts to get activated.
The instant variety, on the other hand, is ready to use without any activation. Though both the type of yeasts can be used interchangeably the quantity that is used will vary as the instant yeast is more concentrated than active dry yeast, and generally needs to be used in smaller quantities.