Ruby Class vs Struct

To add to the other answers, there are some things you can not do with a Struct, and some than you can.

For example, you can not create a Struct with no arguments:

Bar = Struct.new
=> ArgumentError: wrong number of arguments (given 0, expected 1+)

Bar = Struct.new(:bar)
bar = Bar.new(nil)
bar.class
=> Bar

However, a class will let you do that:

class Foo; end
foo = Foo.new
foo.class
=> Foo

You can not set a default value for Struct arguments:

Bar = Struct.new(bar: 'default')
=> ArgumentError: unknown keyword: bar

Bar = Struct.new(bar = 'default')
=> NameError: identifier default needs to be constant

But you can do it with a class, either passing a hash, were the arguments can be in any order or even missing:

class Bar
  attr_reader :bar, :rab
  def initialize(bar: 'default', rab:)
    @bar = bar
    @rab = rab
  end
end

bar = Bar.new(rab: 'mandatory')
bar.rab
=> 'mandatory'
bar.bar
=> 'default'

bar = Bar.new(rab: 'mandatory', bar: 'custom_value')
bar.rab
=> 'mandatory'
bar.bar
=> 'custom_value'

or passing the values directly, were the arguments should be given in the same order, with the defaulted ones always at the end:

class Bar
  attr_reader :rab, :bar
  def initialize(rab, bar = 'default')
    @rab = rab
    @bar = bar
  end
end

bar = Bar.new('mandatory')
bar.rab
=> 'mandatory'
bar.bar
=> 'default'

bar = Bar.new('mandatory', 'custom_value')
bar.rab
=> 'mandatory'
bar.bar
=> 'custom_value'

You can not do any of that with Structs, unless you set default values for your arguments in this super verbose way:

A = Struct.new(:a, :b, :c) do
  def initialize(a:, b: 2, c: 3)
    super(a, b, c)
  end
end

(example taken from this answer)

You can define methods in a Struct:

Foo = Struct.new(:foo) do
  def method(argument)
    # do something with argument
    end
  end
end

Structs can be useful to create data objects, like the point example mentioned in one of the answers.

I sometimes use them to create fakes and mocks in tests in a simple way. Sometimes RSpec allow(foo).to receive(:blah) etc. can get a bit too verbose and using a Struct is much simple.

Struct is a Ruby shorthand for creating Classes. Using Struct where applicable simplifies your code. There is a good discussion of this at https://www.rubytapas.com/2012/11/07/episode-020-struct/

Some of the qualities of using the tool Struct instead of creating a ruby class are the following:

1. A ruby class would take 8 lines to be defined with all the necessary attributes accessors and constructor method and the Struct only take you one line of code

2. After assigning a Struct to a variable I can get and set values using Hash-like subscript syntax and inside it, I can use symbols or strings interchangeably as keys

3. Struct have a equality operator. Struct defines it so that instances with equal attributes are considered equal

4. Unlike attributes defined with attr_accessor Structs can introspect and iterate their attributes, using methods such as members (that will return the instance attributes), each or each_pair (in which you can iterate over the name of the attribute and it's value)

5. Structs also include enumerables so we have a full complement of enumerable methods as well

Source: Graceful dev website

In terms of system performance, this article affirms that classes are faster than structs

I'd like to to @sam_forgot suggested benchmark. The comparison is not very fair. Both class and struct, these days, support keyword arguments. Using keyword arguments on each has opposite effects, as you can see from my example struct's with keyword arguments performance is not that dramatically different from the class.

require 'benchmark'

REP=1000000

SUser = Struct.new(:name, :age)
SUserK = Struct.new(:name, :age, keyword_init: true)

DATA = { name: "Harry", age: 75 }
DATA2 = DATA.values

class CUser
  attr_accessor :name, :age
  def initialize(name, age)
    @name = name
    @age = age
  end
end

class CUserK
  attr_accessor :name, :age
  def initialize(name:, age:)
    @name = name
    @age = age
  end
end

Benchmark.bmbm do |x|
  x.report 'Struct create and access, without keyword arguments' do
    REP.times do
      user = SUser.new(DATA)
      "#{user.name} - #{user.age}"
    end
  end

  x.report 'Struct create and access, with keyword arguments' do
    REP.times do
      user = SUserK.new(**DATA)
      "#{user.name} - #{user.age}"
    end
  end

  x.report 'Class create and access, without keyword arguments' do
    REP.times do
      user = CUser.new(*DATA2)
      "#{user.name} - #{user.age}"
    end
  end 

  x.report 'Class create and access, with keyword arguments' do
    REP.times do
      user = CUserK.new(DATA)
      "#{user.name} - #{user.age}"
    end
  end
end

Rehearsal ---------------------------------------------------------------------------------------
Struct create and access, without keyword arguments   3.484609   0.011974   3.496583 (  3.564523)
Struct create and access, with keyword arguments      0.965959   0.005543   0.971502 (  1.007738)
Class create and access, without keyword arguments    0.624603   0.003999   0.628602 (  0.660931)
Class create and access, with keyword arguments       0.901494   0.004926   0.906420 (  0.952149)
------------------------------------------------------------------------------ total: 6.003107sec

                                                          user     system      total        real
Struct create and access, without keyword arguments   3.300488   0.010372   3.310860 (  3.339511)
Struct create and access, with keyword arguments      0.876742   0.004354   0.881096 (  0.903551)
Class create and access, without keyword arguments    0.553393   0.003962   0.557355 (  0.568985)
Class create and access, with keyword arguments       0.831672   0.004811   0.836483 (  0.850224)

There is quite a big practical performance difference, example behavior in ruby 2.6.3p62:

                          user       system     total     real
Struct create and access  3.052825   0.005204   3.058029  (3.066316)
Class create and access   0.738605   0.001467   0.740072  (0.743738)

Example code:

require 'benchmark'

REP=1000000
SUser = Struct.new(:name, :age)
DATA = { name: "Harry", age: 75 }

class User
  attr_accessor :name, :age
  def initialize(name:, age:)
    @name = name
    @age = age
  end
end

Benchmark.bm 20 do |x|
  x.report 'Struct create and access' do
    REP.times do
      user = SUser.new(DATA)
      "#{user.name} - #{user.age}"
    end
  end
  x.report 'Class create and access' do
    REP.times do
      user = User.new(DATA)
      "#{user.name} - #{user.age}"
    end
  end
end

Think of Struct as a more high-level thing than a Class.

Depending on your goal whether you aim for internal simplicity or the practical power, you chose between Class that has no much magic and Struct that has all those additional fancy methods to work with items.

And only if you are still not sure, then look at the points others have mentioned: having attributes inaccessible by default, inheritance, expandability the attributes list.