To get a good, detailed and relevant to you answer to this you definitely need that degree in Business Studies (with tax accountancy) I mentioned to you recently !
Basically your local tax laws allow some items as allowable expenditure which may be offset against income to reduce your net taxbill. What these items are and how much of them you can claim against tax differs hugely from one country to another, as do tax rates. Which is why only local knowledge is relevant. I have some knowledge of the UK tax laws but I haven't a clue about tax law in Bulgaria.
If you happen to be an internationalist with assets and income in more than one country then you don't have to worry about it, you hire a good firm of tax accountants and let them do the worrying in return for their annual bill !