An SEO agency may include SEO as a standalone service or even offer a broader marketing campaign as a professional SEO may even require a switch over to the HTML source code of a website and some of the tactics may even include web page development as well as design.
There are several tactics and techniques one needs to be aware of used by so called SEO agencies who will claim that they will enhance your website and promote your services and products. If an agency tells you that they uses some of these methods such as include link frames, article spinning, spam dexing, keyword stuffing then avoid those types of agencies as many of the search engines look for websites that employ these methods and remove them from their indices.
These types of tactics and methods are fast becoming a thing of the past whereby keywords are no longer in the control of webmasters which now limits nearly all rank manipulation as well as abuse. Nowadays search engines have been adapted to ensure that the pages show only the most relevant search results as opposed to pages crammed with hundreds of keywords used by bogus webmasters.
A search engines success and popularity is determined by its ability to produce the most important results to any given search. More complex ranking algorithms have been developed by search engines which now make it far more difficult for any webmaster to tamper with. Tools such as Meta tags, site structure and headings were developed which now enables Google to filter and kind of manipulation picked up in the various search engines.
Now that you are aware of some of the methods and tactics used by an unorthodox SEO agency you will save yourself the frustration, time and money of signing up with such agencies as there are many dynamic agencies that are capable of delivering quality services.