Perineal Tears

The Royal College of Obstetricians & Gynaecologists have a full medical description of the different severities of tear, including diagrams and links to more in depth articles

Tomm’y PregnancyHub as a much less clinical presentation of the information, broken down bit by bit and with links when it crosses over with other topics

Parents.com have a more brief look at the 4 types of perineal tear, much better for folks who want to be aware of the possibility without too many details