To get the area between two curves, we slice the region between them into vertical strips, each of width $\Delta x$. Denote by $H(x)$ the height at a point $x$. Since the area of each strip is roughly $H(x)\cdot \Delta x$, the total area is $\displaystyle\sum_{i=1}^n H(x_i)\, \Delta x$.
Taking a limit, the area becomes $$\displaystyle\int_a^b H(x)\, dx.$$
Notice that sometimes we are given the beginning and ending values of $x$ explicitly, whereas others we have to figure out where two curves meet.