Slope Intercept
y = mx + b
m= slope
b= y=intercept

Point-Slope
y - y1 = m (x-x1)
Nothing is substitued for the x and y.  You just plug the coordinates into x1 and y1 and simplify the equation.

Standard Form
ax + by = c
a = slope
b = coeffient of y
c = y-intercept