For a business owner who has decided to boost the rankings of their website to increase the amount of organic traffic they receive, the way to achieve that is via SEO. Assuming that they are not an SEO expert, then they have two choices as to who it is they employ to plan, create, and implement an SEO campaign. Those two choices are either an SEO consultant or an SEO agency.
The most obvious difference between the two is that an SEO consultant will be a single person who operates on a freelance basis. Alternatively, an SEO agency will be a business with employees that combine their efforts to achieve what the client has asked them to. There are several other differences, and these will greatly influence your experience of using them.