What are the functions of steroids?
I am going to assume that your question is referring to natural steroid molecules that our bodies make and use. Steroids are a type of lipid molecule made up of 4 fused carbon rings. Different steroids vary slightly in their structure, but they all have 4 fused carbon rings.
The most common steroid in our bodies is cholesterol. Cholesterol functions to stabilize the cell membrane and make it more resistant to changes in temperature. Our cell membranes need to maintain a consistent fluidity, and cholesterol helps with this as it is embedded in the cell membrane among the phospholipids. When the temperature increases cholesterol helps keep the membrane from becoming too fluid and falling apart, and when the temperature decreases cholesterol helps keep the cell membrane from becoming too rigid.
Other steroids are essentially a modified version of cholesterol and act as hormones. Other common steroids in humans are estrogen and testosterone. Progesterone and cortisol are also steroids that act as hormones. Since these hormones are lipid soluble, they can easily enter cells, move through the nuclear membrane, and act directly on the DNA by turning parts on or off. In this way, estrogen and testosterone are able to travel long distances in the body and affect cells that are far from where the hormones originate in the ovaries and testes.