It used to make a difference when you bought or sold stock which one of these you had. Now it doesn't. If you own 150 shares, you have a round lot of 100 shares and an odd lot of 50 shares. Again, it doesn't matter when it comes time to sell the stock.
Many investors wind up with odd lots because of stock splits.
If you own 100 shares, and the company announces a 3 for 2 split, you'll then find an extra 50 shares in your account on the pay date. That's because there was more work involved in getting you a certificate. Now with the settlement of all trades through a common clearing house, and the vast majority of stock being owned electronically by customers, rather than by certificates, brokerage firms don't have any additional work to fill your odd lot orders. If you want to buy 1 share or 99 shares, it doesn't matter to your broker. The same is true if you want to sell 1 or 99 shares.