If you have a foreign bank account and it is over $10,000 (can't remember the exact number) then you need to complete some special forms.
Tax law says you get taxed on worldwide income (you get worldwide deductions also). If you buy/sell property anywhere throughout the world then you need to report the capital gain on your us tax return.
If you make interest in a foreign savings account; you need to put that on your tax return in USA.
If you own more then 10% of a foreign company you need to complete a special tax form and send it to IRS in philadelphia (it is very complicated form); if you don't do it then there is $10,000 flat penalty.
I see a lot of people owing or stating up companies in india; branch offices; back office) playing around with expense deductions, creating invoices, etc. and taking tax deductions on us tax returns. Not at all smart moves. If IRS comes and audits it will cause big issues.
Note: Many people do this and do not know there are repurcussions. There is ways to do it but most people don't have the knowledge or are too sloppy to do it the right way. Plus it costs some big money to pay to service providers here to show you how to do it the right way. Right now people are getting away with it because it isn't on IRS focus; it only gets on IRS focus when they do a random audit.
However; I see that people are getting more bold; there are conferences, advertisements telling people to invest in properties back home; savings vehicles, etc. As it gets more in the open; eventually someone from federal government; irs will start probing this.