A tanker is a ship designed to transport liquid cargo (such as oil and other petrochemicals) in bulk. Tankers load their cargo by gravity from the shore or through shore pumps and discharge using their own pumps. Oil tankers vary in size, ranging from small coastal ships of 1,000 DWT to medium-sized ships around 60,000 DWT, all the way up to the giant ULCCs (Ultra Large Crude Carriers) of over 320,000 DWT.

Depending on the size of tanker ships, the following sub-segments can be identified:

• ULCC (Ultra Large Crude Carriers): Tankers of 320,000-565,000 DWT. ULCC (Ultra Large Crude Carriers) are designed to carry only crude oil and are challenging to navigate into ports when fully loaded.
• VLCC (Very Large Crude Carriers): Tankers of 200,000-320,000 DWT. VLCC mainly transport crude oil on long-haul routes from the Middle East and West Africa to the Far East and North America.
• Suezmax Tankers: Tankers of 120,000-200,000 DWT. Suezmax Tankers predominantly carry crude oil, though they occasionally transport oil products when their tanks are coated. These tankers are the largest allowed to pass through the Suez Canal in a loaded condition. Known as “one million barrel ships” because of their oil-carrying capacity.
• Aframax Tankers: Tankers of 80,000-120,000 DWT. They generally transport crude oil but can also carry oil products when their tanks are coated. An Aframax Tanker has the capacity to carry approximately.
• Panamax Tankers: Tankers of 60,000-80,000 DWT. They carry both crude and oil products, with most of them being coated. They are the largest tankers allowed to pass through the original Panama Canal when fully loaded.
• Handysize Tankers: Tankers of 10,000-60,000 DWT. Such tankers include those that transport products, chemicals, and other specialized liquids. Since the 2000s, vegetable oil has become a common product transported by Handysize Tankers.
• Small Tankers: Tankers of 100–10,000 DWT that carry oil products, chemicals, or other specialized liquid cargoes.

Based on the type of cargo, the following Tanker Categories can be identified:

• Crude Tankers: Crude Tankers consist of all uncoated tankers over 60,000 DWT that transport crude oil or dirty petroleum products.
• Product Tankers: Product Tankers are equipped with coated tanks (commonly epoxy, less often zinc or polymer), allowing them to carry refined petroleum products (both Clean and Dirty Cargoes). Product Tankers typically range up to the Aframax size of 120,000 DWT, with some coated Suezmax Tankers reaching up to 160,000 DWT.
• Chemical Tankers: designed to carry chemicals, equipped with either stainless steel or coated tanks. These coatings can include epoxy/phenolic or Zinc/Marine Line coatings. Chemical Tankers are capable of transporting a wide range of petrochemical products simultaneously in separate holds (parcelling). Their size typically ranges from 10,000-60,000 DWT, though smaller chemical tankers for coastal trade can carry less than 10,000 tones.
• Specialised Tankers: These vessels are designed for transporting specific liquid cargoes other than chemicals. Examples include Asphalt Carriers, Bitumen Carriers (200-30,000 DWT), Shuttle Tankers (35,000-120,000 DWT), and Small Tankers (100-10,000 DWT) that carry products like water, wine, edible oils, waste, slops, sulphur, methanol, and palm oil. Gas Carriers, such as those used for LNG and LPG, and offshore vessels may also be considered Specialized Tankers.