Crop-lien system was a way for farmers to get credit. After the crop was harvested they would use it to pay back their loan. This is different from sharecropping.
In the postwar South, many former slaves remained as farmers. Not having any money, they could not buy land but instead worked a small portion of a large parcel owned by a single person. Many former slaves were tenants of the same landowner and each had their own section of farm to work on independently, hence the term "sharecropper." In exchange for working on the owner's land, the sharecropper would give some of his harvest as payment.
Also having hardly any possessions, the sharecroppers bartered with merchants to loan them supplies essential for farming. The deal was similar to the land owners': a percentage of the harvest would be given to the merchant to pay for the supplies. This system would have worked if the merchant lenders charged reasonable interest rates. However, racism was a factor in that the merchant's interest rates were impossible for the sharecroppers to pay off. In addition, when the bank gave someone a loan, it was allowed to dictate the crop that a farmer grew. Therefore, the banker for a certain area would tell everyone to grow the same crop and with an increased competition it was even harder for the farmers to sell their goods. It was a constant cycle of debt.
(A farmer would farm someone else's land and give a portion of his crops to the land owner (Sharecropping) with a hope of someday saving up enough to buy the land from the owner. A Sharecropper would have to buy his own equipment and seed with yet more portions of his crop, and usually earned barely enough to survive, falling deeper and deeper into debt.)