[from IBD: Should ETFs Be In Your Investment Portfolio?]While ETFs have existed for several decades, their popularity and diversity have exploded in recent years. But many investors are just learning about them. Those looking for tips on how to invest in ETFs just need some basic information to get started. In basic terms, ETFs can be thought of as mutual funds that trade like stocks. But their portfolios can contain U.S. and foreign stocks, bonds, futures, physical commodities or currencies. And ETFs choose the securities for their portfolios with an investment objective in mind. These investment goals include growth...