This is quite a question because the "medieval" time period encompasses over one thousand years in Europe! That involves countless artists and many art movements!
The medieval art of the Western world covers a vast scope of time and place, over 1000 years of art in Europe and at times the Middle East and North Africa. It includes major art movements and periods, national and regional art, genres, revivals, the artists crafts, and the artists themselves.
Still, I will take a crack at this and say that the major periods are Early Christian Art (Icons), Byzantine Art, Romanesque Art, and Gothic Art. Each have their own particular nuances. However, since you specifically asked about "forms," let's focus on that.
If we could transport back to Medieval times, we would notice many forms of art: tapestry, manuscripts, icons, stained glass, embroidery, mosaics, metalwork, painting, carvings, etc.
With some certainty, you can be sure the focus of most any Medieval art piece will be religious in nature. Why? Most of the focus was on the Christian religion because the artists were very influence by the heritage of the Romans, even though the Roman Empire already fell.
Medieval art in Europe grew out of the artistic heritage of the Roman Empire and the iconographic traditions of the early Christian church.
Another thing you can be sure of is that the art of the Medieval time period ended with the art of the Renaissance time period and its return to classical methods.
There were many Medieval Forms of art. Although the Medieval Period lasted over quite some time, (leading to different styles in artwork), here is a general list of the basic medieval forms of art:
- Illuminated Manuscripts (Embellished religious texts, incorporation of rich colours - Such as silver & Gold).
- Metalwork (Metalworks enhanced advanced skill regarding creating objects relating to adornment, e.g Religious artefacts. Experts productions consisted of jewlery/church doors/sculptures etc.)
- Silversmiths/Goldsmiths (They produced different intriguing types of jewelry.)
- Paintings (Artists typically endured skill in Iconography, Fresco & specialised in religious paintings at the time.)
- Fresco (A form of painting, generally being plaster on walls. They were features of churches in italy & in the Middle ages.)
- Ceramics (Hand sculpted rather than wheel turned products. Mainly cookings pots/jars/jugs.)
- Mosaics (An image that consisted of small pieces of either stone or coloured glass.)
My answer was a brief summary of the website below.