let vs def

1_let.rb

desc "A user's comment" do
  let(:user) { User.create! name: "John" }
  let(:comment) { user.comments.create! }

  it "delegates to user's name" do
    comment.name.should eq(user.name)
  end
end

2_def.rb

desc "A user's comment" do
  def user
    @user ||= User.create! name: "John"
  end

  def comment
    @comment ||= user.comments.create!
  end

  it "delegates to user's name" do
    comment.name.should eq(user.name)
  end
end

3_def_condensed.rb

desc "A user's comment" do
  def user; @user ||= User.create! name: "John"; end
  def comment; @comment ||= user.comments.create!; end

  it "delegates to user's name" do
    comment.name.should eq(user.name)
  end
end

@KeysetTS It doesn't become a variable at all, it defines a method with that name at runtime that returns the value returned by the block given, which is then cached.

let(:user) { User.new }

has the exact same end result as

def user
  @user ||= User.new
end

As for when it is evaluated, the object isn't instantiated until you call user unless you use the let! form, which is evaluated immediately upon entering every test within the describe or context block in which that it is defined. Sure, it's a little advanced and it's a bit of an acquired taste, but the conciseness is always nice when you're writing dozens of them. :-)