Alkyd paints are a type of oil based paint. They can be used as a primer, undercoat and topcoat. Two examples of alkyd based Jotun paints are:
Alkydprimer - a primer for for use above the waterline on steel or existing paintwork (provided that the paint is in good condition- remember if the underlying paint flakes off it is going to take your lovely new finish with it). Alkydprimer can also be used as a topcoat if a matt finish is required.
Pilot II - a topcoat that can be applied directly on to existing oil based paint or over a suitable primer. Pilot II produces a high gloss finish and it is available in virtually any colour. It is for use above the waterline only.
Alkyd paints are also known as oxidative paints because they react with oxygen to form a hard enamel film. This reaction can take a long time (in some cases several months) and so when applying alkyd paints it is important that they are not applied to thickly. Several thin coats are much preferable than one thick one.
A limitation of alkyd paints is that they aren't especially resistant to chemicals or strong solvents. The advantages are that the long drying time means they can be applied easily by hand, they are relatively inexpensive and are easy to touch up.
Modified alkyd paints have been around for many years and although the more modern topcoats provide a more durable finish the ease of application and the reduced cost mean that alkyd paints aren't going anywhere fast.