Fineliner pens are a class of fine fibre- or plastic-tip pens that are typically used for graphic, drawing or sketching purposes, but are also popular for handwriting, as many people like the unique feel of the tip compared to a traditional ball-tipped pen. The tips are generally long and metal-clad to allow use with rulers and templates without bending.
Fineliners are generally relatively cheap, as the construction is very simple. They're mostly disposable, but there are a few premium refillable options. Most fineliner pens use dye-based ink, which is not hugely permanent, but there are many that use lightfast and waterproof pigment ink, especially the more technical drawing ranges, and the Schneider Xpress, which is a lovely premium fineliner.
Most fineliner models are available in several ink colours, and many are available in large ranges of 25 or more colours - the classic here being the Stabilo point 88 in its distinctive orange and white stripes.