There is huge difference between SEO and SEM. In fact there is no similarity between SEO and SEM.
SEO is ranking your site well in the organic search results using tips liked by search engine but SEM is much more bigger than SEO. SEM includes the complete SEO process but also include the PPC Marketing as well as collecting data for more effective SEO.
Lets take an example with some numbers.
Say I want to rank on top for a keyword XYZ. Its highly competitive and requires lots of effort and money to rank well but what would be my ROI. Do I know that no. I can only estimate but what if I do a PPC marketing to rank at top for that keywords and see how the ROI comes to my website.
If now after doing ROI I know what would I make and so now I easily estimate if its worth investing in SEO or not. So SEM is something which you should work on before SEO.