I'm not an expert but what my tax person told me was ur not taxed until u withdraw the money from the account. So if u bought company X and sold to buy into MINE then whatever profit u made off of X isn't taxed. I've kept it pretty simple I haven't "cashed" out completely since I put in my initial "stock allowance" therefore I've not had to pay taxes yet. When I cash out I'm taxed on my profits that I made from my initial investment. If I invested 10k into the market and made 50k and withdrew that money from the market completely I'd be taxed on 40k Now the difference between the high and low tax amounts is if u held the stock for a yr. If so ur taxed less than if u held for six months. That applies to the 40k in my example above. If I'm mistaken someone will no doubt correct me