K2 (spice, etc) is the synthetic cannabinoids (m.psychonautwiki.org/wiki/Synthetic_cannabinoid), which are often fairly dangerous, but are unrelated to kratom.

Kratom (en.wikipedia.org/wiki/Mitragyna_speciosa) is a plant that produces two substances (mitragynine, 7-hydroxymitragynine aka 7-OH) that act like opioids. The pharmacology is relatively unusual, relative to most opioids, as the primary active substances in kratom also bind to serotonin and norepinephrine receptors; other alkaloids in the plant also act as ex. NMDA antagonists which may cause a dissociative experience (vaguely along the likes of ketamine, etc).