The Grange, Farmers' Alliances, and Populist Party are expressions of populism in American political culture. Populism can be characterized in general terms as concerned with the economic and political rights of the rural poor. These movements were fundamentally "grassroots" and derived platforms and purpose from their constituency.
The Grange and Farmers' Alliances were particularly concerned with fighting the domination of American agriculture by large business interests. The Grange was essentially a fraternal order and farmers' union that sought to protect the interests of small family farms through community and collectivism. The Farmers' Alliance had similar goals and motivations and was divided into Northern and Southern wings. The Alliances sought to protect small farms from the predatory practices of big business, which had become increasingly problematic in the postbellum period because of the development of the railroads.
These interests manifested on the national stage in 1891 with the Populist Party, which played a major role in changing political culture in the late-nineteenth-century US. The Populist Party fused with the Democratic Party in the national election of 1896, and their "populist" candidate was the charismatic master orator William Jennings Bryan. The heart of the Democratic platform in 1896 was "bi-metalism," or "Free Silver," which challenged the economic dependence on the gold standard. This was an effort to transform trade and heal the effects of the economic downturn of the 1890s. Populism waned after 1896, but its ideals found further expression in the Progressive movement of the early twentieth century.
The Grange, the farmers' alliances, and the Populist Party tried to help farmers in several ways. The Grange tried to help farmers by providing educational and social events. Farmers would attend classes to learn about new farming techniques. They would attend social events to deal with the isolation the farmers faced.
The Grange and the farmers' alliances also worked to raise crop prices. They tried to pool their crops to help raise the price they got for them. They also tried to work with the railroads to get rebates on shipping their crops. However, they were unsuccessful in doing this.
The Populist Party worked to change financial policy in our country. They wanted a money supply based on gold and silver. This would lead to some inflation that would allow for the prices of crops to rise. The Populist Party also wanted to control the banks and the railroads. They felt the banks charged farmers higher interest rates than other businesses. They also felt that railroads discriminated against farmers by not giving them rebates like railroads gave to other businesses. They wanted to have the government control the banks and the railroad companies.
Movements like the ones you mentioned tried to help farmers by pushing for regulation of businesses like the railroads and by trying to give the farmers more power in the market economy.
The Grange, for example, tried to get farmers to pool their resources and buy and sell cooperatively. If they would all buy together, they might get better deals from the firms that were selling things like fertilizer and plows.
As another example, both the Grange and the Populists pushed for regulation of railroad rates. They didn't want the railroads to be able to (in their eyes) overcharge farmers for hauling produce to market.
These movements, then, were generally aimed at trying to help the farmers to avoid being exploited by big businesses.