RSpec の書き方, メソッド単位に describe で分割するパターンと, オブジェクトの状態単位に context で分割するパターンとあると思う.

marker_spec_01.rb

# メソッド単位に describe で分割するパターン

require 'spec_helper'

module Codebreaker
  describe Marker do
    let(:secret) { '1234' }

    describe '#exact_match_count' do
      subject { Marker.new(secret, guess).exact_match_count }

      context 'no matches' do
        let(:guess) { '5555' }
        it { should == 0 }
      end

      context '4 exact matches' do
        let(:guess) { '1234' }
        it { should == 4 }
      end

      context '4 number matches' do
        let(:guess) { '2341' }
        it { should == 0 }
      end
    end

    describe '#number_match_count' do
      subject { Marker.new(secret, guess).number_match_count }

      context 'no matches' do
        let(:guess) { '5555' }
        it { should == 0 }
      end

      context '4 exact matches do' do
        let(:guess) { '1234' }
        it { should == 0 }
      end

      context '4 number matches' do
        let(:guess) { '4321' }
        it { should == 4 }
      end
    end
  end
end

marker_spec_02.rb

# オブジェクトの状態単位に context で分割するパターン

require 'spec_helper'

module Codebreaker
  describe Marker do
    context 'has "1234" as the secret' do
      let(:secret) { '1234' }
      subject { Marker.new(secret, guess) }

      context 'and has "5555" as the guess' do
        let(:guess) { '5555' }

        its(:mark)               { should == '' }
        its(:exact_match_count)  { should == 0 }
        its(:number_match_count) { should == 0 }
      end

      context 'and has "1234" as the guess' do
        let(:guess) { '1234' }

        its(:mark)               { should == '++++' }
        its(:exact_match_count)  { should == 4 }
        its(:number_match_count) { should == 0 }
      end
    end
  end
end