no class definition, the response variable is a continues value. Model the relationship between explanatory variables and the response variable.
Suppose you want to estimate the house price based upon the area of the land, you can use linear regression. It is an equation of a straight line, where your response can take any value between -Infinity to + infinity.
Now suppose you want to fit a model for two class problem like ham or spam,
or malign or benign tumour(ie response variable can only take 0 or 1). here, you got only two values for the response variable. Now, if you want to fit this with a linear model (which as we know can take any value between - inf to + inf) how would be interpret response value between 0 and 1 and how would you interpret values more than 1 an double than 1 and ten times of 1...(you are getting my point?)
hence, in such cases, we tend to use Logit function, which can only take two values.
In this case, we have to decide about the threshold for the decision boundary but once done, you would know your response values as 0 r 1.