Question 386595
I think one is a composition {{{f(g(x))}}} where you substitute {{{g(x)}}} for {{{x}}}. 
The other is an actual multiplication of two functions.
{{{fg(x)=f(x)*g(x)}}}
So if {{{f(x)=x+1}}} and {{{g(x)=x}}}
Then {{{f(g(x))=g(x)+1}}}
{{{f(g(x))=x+1}}}
and {{{fg(x)=(x+1)x}}}
{{{fg(x)=x^2+x}}}