Q:

"covariance" , "covariate", "coefficient of variance"?

Could you please explain what is the difference of these "covariance" , "covariate", "coefficient of variance"?
I would appreciate example from biology if possible

A:

"Covariance" is the raw version of correlation.  It is a measure of the linear relationship between two variables.  For instance, you could measure brain size and body weight (both in grams) across species.  Then you could get the covariance but you would usually want to scale it and get the correlation.
"Covariate" is a variable in a regression or similar model. For instance, if you were modeling number of animals in a given area, you might have covariates such as temperature, season, latitude, altitude, time of day and so on. 
There's no "coefficient of variance" that I know of.  I think you mean "coefficient of variation".  It is simply the  standard deviation divided by the  mean of a variable.  IT can be a useful way to compare the "spread" of two variables, as long as both take only positive values.  For instance, you might want to see if head size varies more in pygmy chimpanzees than in humans. But you'd have to control for the size of the head.  CV is one way to do that. 

