carbocation
/ CAR-bo-CAT-eye-on /
Carbon normally likes to share four pairs of electrons, sitting comfortably with a full octet. A carbocation is a carbon atom that has been left short: it carries a positive charge because it has only six electrons around it, one bond fewer than it wants. It is electron-hungry and desperate, which makes it extremely reactive and very short-lived.
The electron-poor carbon is bonded to only three groups, and to spread those three bonds as far apart as possible it goes flat: the three bonds point to the corners of a triangle at 120 degrees (sp2 hybridized), leaving an empty p orbital sticking up and down like an empty pocket waiting to be filled. Carbocations are not all equally unstable. A tertiary cation (three carbons attached) is more stable than secondary, which beats primary, which beats methyl. The reason is that neighboring C-H and C-C bonds can donate electron density into the empty orbital, both through space (hyperconjugation) and through the bonds (the inductive effect), easing the electron shortage. If a double bond or lone pair sits next door, resonance can stabilize the cation even more dramatically (as in allylic and benzylic cations).
Carbocations are the central characters of much of organic chemistry. They appear in SN1 substitutions, E1 eliminations, electrophilic additions to alkenes, and Friedel-Crafts reactions. Because the more stable carbocation forms preferentially, their stability order is the real reason behind Markovnikov's rule. Being so unstable, they grab the nearest electron source instantly, and they will even rearrange their own skeleton to become more stable.
When tert-butyl chloride (CH3)3C-Cl loses its chloride in water, it forms the tert-butyl cation (CH3)3C+, a flat, three-armed carbon that is relatively stable thanks to its three neighboring methyl groups.
Three methyl neighbors feed electron density into the empty orbital, so this cation forms readily.
Stability order (tertiary > secondary > primary > methyl) is about how easily the cation forms, not about it being safe; even a tertiary carbocation is a fleeting, highly reactive species, not a bottle-stable compound.