Personalised Search

Personalised Search fetches results and delivers search suggestions individually for each of its users based on their interests and preferences. Web users in Russia, for instance, typing on Yandex the query [nevermind] might just as likely be looking for Nirvana's album, as wishing to find out what this word means. Personalised Search would know the difference and would act accordingly.
Yandex’s Personalised Search is based on the user's language preferences, their search history and their clicks in search results. The user's search history tells the search engine what may be currently relevant for this particular user. Someone searching on the internet for free software, books or music is very likely to be interested in this type of content as such. Those users, who frequently visit websites in English, may very well appreciate search results in this very language. As personal preferences tend to change over time, Yandex considers only the relatively fresh search history spanning the period of a few months to offer the users personalised search results and make personalised search suggestions.

Search suggestions

In contrast to regular search suggestions, personalised search suggestions are targeted individually at every single web user. When guessing what a searcher might want to find, Yandex suggests potential search queries, based on what other people with similar online preferences looked for on the internet. The search engine classes everyone into one of about 400,000 user groups with more-or-less common interests. This classification is fluid – it changes for every user according to the changes in their online behaviour.
In practice, web users repeat about 25% of their search queries and often click the same search results. This behaviour can be interpreted as going to frequently visited websites or viewing popular or personally relevant web documents. Yandex offers users a shortcut to favourite content by showing them recently made queries and their favourite websites in the search suggestions when they type the first letter of their new search.
When choosing search suggestions for a specific user, Yandex also looks at what searches have been previously made during a whole search session. So, the search engine would know that Christopher Lloyd would probably be a better search suggestion for 'c' in the search box than any other if the searcher has looked for Back to the Future before.

Personal results

Other than making personalised search suggestions, Yandex helps its users to achieve their search goals by providing them with the most relevant search results. In doing that, the search engine uses a special personalised ranking algorithm, which it recalculates according to the ever-changing interests and language preferences of each user.
Personalised ranking algorithm allows the search engine to understand how well each of the fetched results matches the user's expectations. Search results are evaluated and ranked according to their usefulness for a particular web user. The same search query made by two different people would trigger the same results ranked differently to match their individual interests. An inveterate gamer and an art film enthusiast, for instance, will see on top of search results links to websites that are relevant to their respective interests even if they both look for 'Stalker'.
Yandex's ranking algorithm takes into account long-term, medium-term and shot-term interests of each user. All of these affect search results in one way or another. Long-term interests reflect the user's language, location, demographics, permanent needs or preferences, while short-term interests express what is important for the user right now. Even if you are a more-or-less always interested in music and film, you may unexpectedly find yourself looking for a computer game online and that'll be just once. Search interests that come and go account for more than half of all searches on Yandex.
To keep updated on the user's long-term interests, Yandex analyses the two past months and the past week of their search history. Short-term interests are being tracked in real-time by looking at current search sessions, which allows the search engine follow the user's search intents. When someone with a long-term affinity to reading makes a search with the name of a popular movie listing website, Yandex can confidently assume that this person is looking for a film adaptation:

Yandex uses its own data delivery technology for real-time query processing. This technology allows collect data and send it over to the search engine every seven seconds, so that each click or search that the user makes on Yandex is taken into account and directly affects search results.
Personalised search is enabled by default for every more-or-less frequent search user. The more queries the user makes, the better results and suggestions the search engine can provide. Personalised search deactivates if there aren't enough searches on which personalisation can be based, and activates again when queries start coming. Personalisation can also be enabled or disabled manually in the search engine's settings.
Currently, personalisation on Yandex works best for searches in Russian, but as the search engine accumulates search statistics in other languages other users will also be able to fully enjoy it.