In regards to the anime Bleach], a shikai is the first release of a zanpakuto, the sword used by shinigami]. Each shikai has a special ability.
To release the shikai form of a zanpakuto, one must call it's name (eg; "Scatter, Senbonzakura", "Awaken, Benihime", "Dance, Sode no Shirayuki"). This is no longer necessary when one has attained bankai, the second release.
Some shikai releases are...
Hitsugaya's Hyourinmaru- takes the form of an ice dragon
Renji's Zabimaru- a sword that can be extended into different segments
Ichigo's Zangetsu- a gigantic butcher-knife type sword
Matsumoto's Haineko- a disintegrated sword
Kira's Wabisuke- turns into a square spiral