The DrawF commands draws a single expression on the graph screen in terms of X using Function graphing mode, irregardless of what graphing mode the calculator is actually in. For example, DrawF X2 will draw a [wikipedia:parabola parabola] in the shape of a U on the screen. Of course, how it is displayed all depends on the window dimensions of the graph screen; you should use a friendly window to ensure it shows up as you intend.
When evaluating the expression using DrawF, the calculator will ignore the following errors: ERR:DATA TYPE, ERR:DIVIDE BY 0, ERR:DOMAIN, ERR:INCREMENT, ERR:NONREAL ANS, ERR:OVERFLOW, and ERR:SINGULAR MAT. If one of these errors occurs, the data point will be omitted.
For this reason, DrawF can sometimes behave in an unexpected fashion: for example, it doesn't throw an error for list or matrix expressions (it won't graph anything, either).
You can use DrawF to draw an expression instead of having to store an expression to a Y# variable and then displaying it. At the same time, if you plan on manipulating the expression (either changing the value or changing the expression itself), it would be better to simply use the Y# variable.