Adipocytes, are fat cells. There are two types of fat cells:

White Fat Cells, which are the most common type in adults. These are unilocular (have a single large lipid droplet), have a large diameter (100µm or more), and are found in subcutaneous, omentum & mesentry regions.

Brown Fat Cells are multilocular (each cell has many small lipid droplets). They are smaller than white adipocytes. They are present in large amounts in the new-born & hibernating animals. In adults restricted to areas around kidney, aorta and regions of the neck and mediastinum.

Fat cells are thought to develop from fibroblast like cells. The fat/lipid droplets co-alesce into a large droplet, leaving only a thin rim of cytoplasm, and the nucleus is pushed out to the side.

When tissue is fixed and stained for histology, the single large lipid droplet is extracted, and the cells that remain look 'empty', as shown in the picture and diagram here.