What is the relationship and difference between tox and Travis-CI?

I am confused about the conceptual relationship/differences and usage scenarios between tox and Travis-CI.

I saw Python projects are testing for different Python environments by using tox OR Travis-CI respectively. So it seems that these two CI services can be the alternative to each other.

But I also read that we can run tox with Travis-CI. And this seems that these two CI services are the complementary to each other.

So can anyone make some clarification or point some resources about the conceptual relationship and/or differences and usage scenarios between these two? Thanks in advance!