Question about buying stock and doing a round trip

[Bill Jackson]

what's the difference between buying a stock and selling the same day and buying  a stock and selling the next day or next week. 

if you already own a certain stock, then you buy a few more shares then sell them the next day, does your account keep track of last in ( bought) first out ( sold) ?

lets say i bought 50 shares of stock A at $2 a share a year ago. Then a year later I bought 3 shares for $1.00, then the same day sold the 3 shares for $1.25 . would  it take an aggregate of all my stocks gain or use the last 3 I bought the same day. Or if I sold a few days latter. so in other words since the value of all my stock A had actually declined. Buy 3 shares for $1.00 the selling the 3 shares of stock A would My account reflect a gain of .75