Dear Bilal,

In addition to what Francisco Solano wrote, the following text describes the four classes in details:

Sterols, also known as steroid alcohols, are a subgroup of the steroids and an important class of organic molecules. They occur naturally in plants, animals, and fungi, with the most familiar type of animal sterol being cholesterol. Cholesterol is vital to animal cell membrane structure and function as a precursor to fat-soluble vitamins and steroid hormones.

A steroid is an organic compound with four rings arranged in a specific configuration. Examples include the dietary lipid cholesterol, the sex hormones estradiol and testosterone: and the anti-inflammatory drug dexamethasone. Steroids have two principal biological functions: certain steroids (such as cholesterol) are important components of cell membranes which alter membrane fluidity, and many steroids are signaling molecules which activate steroid hormone receptors. The steroid core structure is composed of seventeen carbon atoms, bonded in four "fused" rings: three six-member cyclohexane rings (rings A, B and C in the first illustration) and one five-member cyclopentane ring (the D ring). Steroids vary by the functional groups attached to this four-ring core and by the oxidation state of the rings. Sterols are forms of steroids with a hydroxyl group at position three and a skeleton derived from cholestane. They can also vary more markedly by changes to the ring structure (for example, ringscissions which produce secosteroids such as vitamin D3).

Triterpenes are a class of chemical compounds composed of three terpene units with the molecular formula C30H48; they may also be thought of as consisting of six isoprene units. Animals, plants and fungi all create triterpenes, with arguable the most important example being squalene as it forms the basis of almost all steroids.

Triterpenes exist in a huge variety of structures with nearly 200 different skeletons known from natural sources or enzymatic reactions. These may be broadly divided according to the number of rings present; although in general pentacyclic structures (5 rings) tend to dominate.

Triterpenoids: by definition triterpenes are hydrocarbons and possess no heteroatoms, functionalized triterpenes should instead be called triterpenoids; however this distinction is not always adhered to in scientific literature, with the two terms often being used interchangeably. Triterpenoids possess a rich chemistry and pharmacology (e.g. Cholesterol) with several pentacyclic motiefs; particularly lupane, oleanane and ursane showing promise as anti-cancer agents.

Hoping this will be helpful,

Rafik

Steroids: Any compound composed of at least seventeen carbon atoms in carbon-carbon bonds forming four fused rings. Three cyclohexane rings (A, B, and C) form the skeleton of a perhydro phenanthrene and ring D is a cyclopentane. Steriods are derived of a cyclopentaneperhydrophenantrene.

Sterols: A steroid with a alcohol function. e.g. cholesterol

Triterpene: C30 hydrocarbons without heteroatoms and derived from six units of isoprene. It can be linear.  Scualene is a triterpene, still not cycled

Triterpenoids: Similar to triterpenes, but it can contain  other heteroatoms such as oxygen

Dear Bilal,

In addition to what Francisco Solano wrote, the following text describes the four classes in details:

Sterols, also known as steroid alcohols, are a subgroup of the steroids and an important class of organic molecules. They occur naturally in plants, animals, and fungi, with the most familiar type of animal sterol being cholesterol. Cholesterol is vital to animal cell membrane structure and function as a precursor to fat-soluble vitamins and steroid hormones.

A steroid is an organic compound with four rings arranged in a specific configuration. Examples include the dietary lipid cholesterol, the sex hormones estradiol and testosterone: and the anti-inflammatory drug dexamethasone. Steroids have two principal biological functions: certain steroids (such as cholesterol) are important components of cell membranes which alter membrane fluidity, and many steroids are signaling molecules which activate steroid hormone receptors. The steroid core structure is composed of seventeen carbon atoms, bonded in four "fused" rings: three six-member cyclohexane rings (rings A, B and C in the first illustration) and one five-member cyclopentane ring (the D ring). Steroids vary by the functional groups attached to this four-ring core and by the oxidation state of the rings. Sterols are forms of steroids with a hydroxyl group at position three and a skeleton derived from cholestane. They can also vary more markedly by changes to the ring structure (for example, ringscissions which produce secosteroids such as vitamin D3).

Triterpenes are a class of chemical compounds composed of three terpene units with the molecular formula C30H48; they may also be thought of as consisting of six isoprene units. Animals, plants and fungi all create triterpenes, with arguable the most important example being squalene as it forms the basis of almost all steroids.

Triterpenes exist in a huge variety of structures with nearly 200 different skeletons known from natural sources or enzymatic reactions. These may be broadly divided according to the number of rings present; although in general pentacyclic structures (5 rings) tend to dominate.

Triterpenoids: by definition triterpenes are hydrocarbons and possess no heteroatoms, functionalized triterpenes should instead be called triterpenoids; however this distinction is not always adhered to in scientific literature, with the two terms often being used interchangeably. Triterpenoids possess a rich chemistry and pharmacology (e.g. Cholesterol) with several pentacyclic motiefs; particularly lupane, oleanane and ursane showing promise as anti-cancer agents.

Hoping this will be helpful,

Rafik

Francisco Solano, Rafik Karaman  bundle of thanks to you both. Indeed its very helpful.

Professionally, it is expected that downvoting an answer should be based on scientific reasons. If the person who downvoted my answer has the courage, he should write the reasons for his downvoting.

Thanks,

Rafik