There are two main kinds of articulation, Hard and Floating.
Two or more pieces of metal joined with a rivet with enough play that the the pieces may rotate. The rivets may sit on holes or slots.
There are two kinds of hard articulation, shell articulation and compression articulation.
Shell articulation sits on the outside of a curve like the outside of an elbow or knee and expands out.
Compression articulation compresses inwards and can usually be found over the inside of joints in high-end late period armours such as Henry VIII's tonlet armour from the Field of the Cloth of Gold.
The pieces move on leathers. Eg: Gauntlet fingers.