These are just a few notes explaining the tier rankings and other considerations for vancian spellcasters.

Vancian spellcasters are those with the "fire-and-forget" mechanic originally inspired by the works of Jack Vance.  This includes all core spellcasters, such as Wizards, Sorcerers, Clerics, Druids, and the like.  Psychic spellcasters also use this mechanic, so Psychics, Occultists, Spiritualists and similar are included here.

Spells are among the most versatile options for any character in D&D 3.5 or Pathfinder, and so spellcasters tend to be of a high tier.

"Prepared" spellcasters must prepare their spells each day, but they enjoy more versatility than "Spontaneous" spellcasters who benefit from fewer spells known.  For this reason, prepared spellcasters tend to be higher tier than spontaneous ones, even when they share the same spell list.

These rankings have actually been discussed in-depth online, so I'll skip over most of the discussions here.

There are a variety of options that can change the tiers of spellcasters, if you examine the tier listings you'll see that a few classes change dramatically depending on their access to different spell lists (or their rather restrictive spell list).  Generally, the size, power, and "prepared" nature of a character's spell lists determines his or her effective tier.  Other considerations, such as D&D 3.5's persistent spell, can dramatically change a character's effectiveness.  Metamagic cost reductions or abilities that allow lower-level spells to mimic higher-level spells are also worth considering.  For example, both the Incantatrix and Shadowcraft Mage are prestige classes that can raise spellcasters to tier 0.  Knowstones and Mnemonic Vestments can allow spontaneous casters to behave like prepared casters, so they usually warrant a tier increase.

Simply increasing the DC of your spells is not sufficient to warrant a tier increase.  In fact, spellcasters are often behind the curve when it comes to spell DCs, so I encourage DC optimization.

2023 Notes: The curve for higher-tiered characters has again been reduced, and this partially reflects the current metagame where Vancian spellcasters are generally countered by newer options, especially at lower levels.

See also 3.P Homebrew Magic Options.