Without complicated business lingo, please tell what numbers to know in order to buy a business?

1. Market size and Company's Market share
2. Turnover trend over years
3. Profit ratio (profit/Turnover) (can tell you how much you can increase your profit if you can increase sales)
4. Return on Equity (profit/equity => gives you an indication of % return on the money you put in the business
(2 & 3 before must be seen in the industry context)
5. Profit trend and profit/share; Operational cash-flow per share (gives you a view of how much cash the business generates, remember profit is vanity, cash is reality!)
6. If you have a price quoted: P/E Ratio => Profit per share/Earnings per share, gives you a view on term you will recover your investment
etc.

There are many numbers that need to be looked carefully, generally when you want to buy a business you would employ a due dilligence exercise that would look not only to the numbers but to many other aspects as possible contingent liabilities not disclosed in the financial statements etc. There is no easy way or magic formula that would give you a value of a company and unfortunatelly all the evaluation methods would use to some extend specialist economic terms.

Hope this helps, however, if you have other more specific questions, please ask.