Certain detrimental spells have 'counters'; if you remove all of the counters, the spell is eliminated. There are five types of counters; detrimental, poison, disease, curse, and corruption Each can be removed by a different line of player-spells. Additionally, the AA ability Radiant Cure removes counters from every category except corruption.
If you have multiple effects with the same counter on you, the effect removed first will be the first spell in your window.
You can tell what counter any spell has by looking it up on Lucy.
Many spells -- especially at lower levels -- have poison counters. Many low level monsters proc poison-based spells; this is especially true of snakes and lizards. Many higher level mobs (especially Raid mobs) have poison-counter aoes.
Poison counters can be removed by several classes -- Clerics, beastlords, paladins, druids, rangers, bards, and shaman all receive spell lines dedicated to removing poison. Additionally, poison spells can be removed by drinking a potion of the Distillate of Antidote line.
One atypical spell that has poison counters is Tash; as the tash line prevents you from entering OOC regeneration, it's handy to remember that poison cures remove it. Because tash is poison-based, it can be very wise for a soloer who cannot cure tash themselves to bring along cure poison potions when they hunt.
Disease is another extremely common counter. Low level undead frequently proc disease-based spells, as do other mobs. Like with poison, disease is a relatively frequent counter for aoes on raids.
Disease counters can be removed by the same classes that can remove poison counters; clerics, beastlords, paladins, bards, druids, rangers, and shaman. Disease counters can also be removed via the Distillate of Immunization line of potions, or the rare but incredibly useful Shield of the Immaculate.
Curse counters were introduced in Luclin, and were originally used for AOE's on high end raid mobs. Curse counters are now more generally found on traps/chests, as well as some raid aoes. Curses are relatively uncommon in groups, but do exist.
Curses can be removed by clerics, paladins, druids or shaman. The main spell used to remove curses by most classes, Remove Greater Curse, is somewhat difficult to get; it can only be obtained from the Rhags in SSRA Temple or from the Takish-Hiz LDON camp.
Detrimental counters are only removable via radiant cure and the shaman spell Pure Spirit. Detrimental counters, unlike all other spell counters, are not listed in a known field in Lucy. Detrimentals are present on almost all spells that do not have other counters; detrimentals are the default so to speak.
Corruption was added with the Serpent's Spine expansion, and is currently only used on extremely highend raids (FC, AG, and Solteris.) Paladins, shaman, druids, bards, and clerics can remove corruption counters.