check the first picture below, that's just an example of the correspondence of those two angles in a right-triangle, notice, the cosine of one is the sine of the other.
now, keeping in mind that
[tex]\bf sin(\theta)=\cfrac{opposite}{hypotenuse}
\qquad\qquad
cos(\theta)=\cfrac{adjacent}{hypotenuse}
\\\\\\
% tangent
tan(\theta)=\cfrac{opposite}{adjacent}[/tex]
check the second picture below.
