A pure is a style of account that only focuses on certain skills to manipulate their combat level. There are many different types of pures; some focus on strength and others can focus on anything from defence to prayer. Another key aspect of a pure is maintaining a reasonably low level skill (the most common being defence) to maintain a lower combat level.
Most pures tend to keep Prayer at a minimum of 43 or 77. This is to prevent combat level gain and maximize the player's damage output.
43 is for protection prayers and the offensive damage bonus prayers.
77 is for access to both of the Dungeoneering exclusive prayers.
Pures that train attack gravitate toward benchmark weapon requirements. Many of which tend to be the requirement for chaotic weapons, Abyssal whip, and dragon weapons.
One Defence Pure
Possibly the most common pure, the one defence pure is an account that maintains a defence level of one. Although having a low defence may limit their ability to defend themselves, it offers up a lot of damage for lower level combat.
One reason for their excessive popularity may be the "::pure" command which grants any player with a maxed pure account if used in Varrock west bank.
A berzerker pure (also known as a zerker) is another type of pure that manipulates defence. Each berzerker pure has 45 defence (high enough to wear a berzerker helmet) and is only dictated by that defence level. No combat stat other than defence plays a role in dictating a berzerker pure.
Although not required, the curse 'Turmoil' is a staple of this type of pure.
Although this type of pure may seem to focus on defence more, it tends to be more strength-based. Commonly used armor for this type of pure is heavy in strength bonuses and these types of accounts generally hit higher than most other pures.
This type of pure has 20 defence and attack and focuses heavily on the use of corrupt degradable equipment such as Statius'. This type of pure carries the highest bonuses for it's level and can hit marginally higher than the one defence pure.
A tank is a pure based on garnering the most defence bonuses while also attacking the enemy with a Summoning familiar. Most summoning tanks plan on achieving 99 defence and 99 summoning, but there are some exceptions. Some tanks plan on getting a defence level high enough to wear a certain type of equipment (the highest defence being 80, in that case, to equip Torva).