Carageenans are extracts from red seaweed appearing in kappa, iota, and lambda forms. Their differences are marked by the number of sulfates they have per saccharide. Kappa has one sulfate, Iota has two sulfates, and Lambda has three sulfates per saccharide. The differing number of sulfates impacts the strength and rigidity of the bonds formed. Kappa forms very strong rigid gels, Iota forms soft gels, and Lambda merely thickens.
The Carageenans are often times used as thickening and gelling agents particularly in products with a high calcium content (like the chestnut cream here). They are found in a variety of products as thickeners and stabilizers (ice cream, salad dressings, processed meats, toothpaste, etc).
In Modernist cooking they are used for their gelling and absorbing power, which is much greater than traditional thickeners like cornstarch and flour. Additionally, they are a vegan alternative to traditional gelling agents. Find all three versions here.