native object
object in an ECMAScript implementation whose semantics are fully defined by this specification rather than by the host environment.
NOTE Standard native objects are defined in this specification. Some native objects are built-in; others may be constructed during the course of execution of an ECMAScript program.
Source: http://es5.github.com/#x4.3.6
host object
object supplied by the host environment to complete the execution environment of ECMAScript.
NOTE Any object that is not native is a host object.
Source: http://es5.github.com/#x4.3.8
A few examples:
Native objects: Object (constructor), Date, Math, parseInt, eval, string methods like indexOf and replace, array methods, ...
Host objects (assuming browser environment): window, document, location, history, XMLHttpRequest, setTimeout, getElementsByTagName, querySelectorAll, ...