Basic Difference between SEO or SEM
SEO - (Search Engine Optimization) - It means of optimizing your website so that you get organic (free) traffic from the search engines.
SEM - (Search Engine Marketing) - It is an online marketing strategy that involves getting traffic, paid and unpaid, from the search engines.
"SEM is a form of Internet marketing that involves the promotion of websites by increasing their visibility in search engine results pages (SERPs) through optimization and advertising."
The answer to that depends on your overall strategy and budget.
If you’re running a startup and bootstrapping, you probably won’t have a lot of money for paid search
If your business has plenty of cash and a narrowly defined target market, you might benefit from paid search