Arrow heads (or points) are the pointy bit at the end of the arrow that's made of metal. Back in the medieval day they had many different names for the different arrow heads and we still do today. The London Museum started cataloging arrow heads found in the 1940 London Blitz and so you might have London Museum Type 9s and London Museum Type 15s thrown around which give us some form of identification. However it was a bit random so in 1996 Oliver Jessop at Durham University published Jessop's Arrow Typology which not only dated when an arrow head was used and also classified them by use. Therefore an MP2 refers to a Multi-Purpose arrow head (used for hunting and war), an H2 refers to a Hunting arrow head and an M2 refers to a Military arrow head. The lower the number next to the letters generally the earlier they were used in the medieval period and the later versions had higher numbers.
If you are looking to put some reproduction medieval arrowheads on your arrows check out Jessop's Arrow Typology before you buy. Find out if your 15th century archer is about to shoot some arrowheads that went out of fashion in the 12th century ...
Link to Jessop's Arrow Typology PDF (2.49Mb)
If you're using reproduction arrowheads write down "reproduction arrowheads" (I'll be asking more questions direct to you later as reproduction arrowheads have differing sizes and may require cutting a shoulder into the arrow shaft.)
Nowadays we use arrow heads that are pointy enough to stick in a target but smooth enough to pull the arrow back out because although it's nice shoot barbed arrows the barbs are designed to catch and stop the arrow from being removed. So today we tend to use "field points" and "bullet point" arrow heads. Fear not thought history lovers because the Field Point is similar to the MP9 (using Jessops Arrow Typology) which was used between the 12th and 15th centuries. Also bullet points are similar to the MP10 which was used in the 16th century.
Modern arrow heads are sized to the arrow shaft they go on i.e. 11/32" but also the weight which is measured in grains (as is the finished arrow). Grains (weight) is an old (very old) measurement that used to sort of mean exactly what it appears to mean - a grain wieght of a grain of wheat or barely. Grains these days have been standardised and along with arrows are used in the weight measurements for gunpowder weapons. Roughly 1 grain is equial to 0.06 grams and arrowheads are available in 100, 125,145 and 160 grains (6.47 grams right through to 10.36 grams). With the weight of the arrowhead you can affect the the arrow's overall weight, centre of gravity and the flight characteristics of the arrow (as does the fletchings and shape of the arrow shaft!). It's all one big tinkering exercise to change the centre of gravity, geometric centre and centre of (upward) pressure to make your arrow fly longer. However for simplicity's sake 125 grain is a nice one to go for at the start. If you want to change your arrowhead weight go for it because there are no charts to look this up and trial & error is sometimes the best way of matching your arrows to your bow...
Write down your arrowhead weight e.g. 125 grain.