I am getting difficulty in understanding the definition of the statistic.

From wikipedia, I come to understand that statistic is any 'information'(range, mean, variance) of any sample of any given population.

Whereas in my college the definition of statistic is given as:

Suppose $(X_1, X_2,... ,X_i,...,X_n)$ is a random sample of size $n$ from any given pdf or pmf. A function $T=t(X_1, X_2,... ,X_i,...,X_n)$ free from unknown parameter is called a statistic.

I cannot understand the definition given in my college. Are the two definitions related to each other? What is the need of finding the statistic?