Slope-intercept form: y = mx + b
Point-slope form: (y - y1) = m(x - x1)
They're both true because they're the same equation, just with stuff moved around a bit. The difference is what each one is useful for.
Slope-intercept form is useful when we know (or want to know) the slope of the line and where it crosses the y-axis (x = 0).
Point-slope form is useful when we know the slope and one of the points on the line.