White Hat SEO
You do not need a hat to hire an SEO but if you do hire an SEO expert s/he should have a white hat.
S/he should wear it while working for you.
White hat SEO is clean user oriented search engine optimization that neither alienates potential clients nor leads to penalties by search engines like Google.
You do not need to trick or game search engines to rank high, get more traffic, conversions and sales.
White hat SEO offers many methods to optimize web sites, gain link popularity and thus high rankings, convert visitors and streamline your sales funnel.
Most notably link baiting, conversion optimization and ROI tracking have to be implemented.