The keywords that you give are searched for in the Title of the job, Key Skills asked for in the job and the entire text of the job description in that order.
The jobs are sorted on relevance by default. The most relevant ones appear at the top.
Company Jobs are posted by corporates trying to fill the positions for their own requirements. Placement Agencies Jobs are those posted by consultants or agencies who have been hired by corporates to fill their requirements. Choose All option for seeing the maximum number of results.