keywords
it is very important one
information can be segregated easily and quickly over web otherwise search engine is compact world

for eg
searching books related to specific subject let take microbiology


information required on cholera

searching steps
step 1
we know the subject which is primary keyword :"microbiology"

step 2
it is bacteria which is considered as secondary keyword

step 3
the bacterium causes diarrhoea so the family is enterobacteriacae

finally the specific word lets take on a toxin created by cholera


so by following these protocol we can get ultimate search results