rspec is a meta-gem, which depends on the rspec-core, rspec-expectations and
rspec-mocks gems. Each of these can be installed separately and loaded in
isolation using require. Among other benefits, this allows you to use
rspec-expectations, for example, in Test::Unit::TestCase if you happen to
prefer that style.

Conversely, if you like RSpec's approach to declaring example groups and
examples (describe and it) but prefer Test::Unit assertions and mocha, rr or
flexmock for mocking, you'll be able to do that without having to install or
load the components of RSpec that you're not using.