Profit arising on transfer of capital asset is called capital gain.
In case of shares if period of holding is less than 1 year then it will be called Short term capital gain.
If period of holding is more than 1 yr then it will be long term capital gain.
In case of listed shares Long term are exempted from tax.
In case of Short term gain you have to pay  @  15% tax after taking advantage of basic exemption limit i.e 150000.
for example if your income is 175000 (incliding 30000 STCG) then your tax will be calculated as follows
(175000-150000)*15%
Always remember 15% is special type of rate which is chargable in case of gain arising from listed equity share(STCG)