Combo search refers to a different way of presenting search results. In the Google world, people are used to see pages after pages full of only web search results, or image search results, or new search results, etc (with the exception of some sponsored links). This is, however, not the way search results are presented in Korea.
Let’s take an example: search for “cat” in Naver, Korea’s top search engine. You’ll see a long results page divided into many different sections, including sponsored results, news results, category results, local results, book results, movie results, blog results, music results, knowledge results (similar to Yahoo Answers), as well as web results. This way, users are exposed many different types of results for that query term, and the user experience is entirely different.
This type of search results display is possible in Korea because of its high broadband penetration rate. With dialup access, this type of display would simply take too long to download.
This is perhaps why Google is not able to gain much in the Korea market. Google is noted for the simplicity of its pages, but this may work against them in Korea, where users have come to expect to see a large variety of pages.
In this type of system, SEO for web pages becomes less important because web results themselves are less important there. Even if you make it to #1 in web search, you might still be way below the fold (depending on where the search engine decides to place the web results section). In Korea, SEO thus means looking into image search, book search, etc, in addition to simply web search.