Things that are endless go on forever, or at least appear to. The ocean looks endless when you sail out of sight of the shore.
Endless things are infinite — your love for someone might be endless, and the mathematical symbol of infinity represents an endless number. You're most likely to use this adjective to describe things that only seem to last forever, like an endless meeting at work or an endless commute during rush hour. The Old English root of endless is endeleas, "boundless or eternal."