There are several types of travel sleeping bag liner or sleeping sheets as sometimes they are called, and as many different materials used, they're basically cotton, silk or poly cotton, some have a pillow slot at the top to take a pillow and others do not,there are mummy shaped and rectangular, the more expensive ones may have antibacterial and insect repellent properties and perform well in hot or cold climates.
They can be used on their own or used in a sleeping bag, for your own comfort and hygiene we'd always advise when you travel to take a sleeping bag  liner
This lifeventure rectangular sleeping bag liner is 100% cotton and is treated with Polygiene odour control technology. The anti-microbial treatment is integral to the fibres within the fabric preventing the build up of odour causing bacteria, keeping your sleeping bag liner cleaner and smelling fresher for longer. The soft, breathable liner is generously proportioned with integrated pillow sleeve and contact germ guard.
Comes with a tough ripstop carry case to stow away easily and protect from the elements.