To specify categories and tags, you should write in YAML front matter. It's so complicated because there are 3 ways to specify it.

This article is targeted on Jekyll 0.12.0.

(1) Use singular key

Use singular key (such as category or tag), then we can specify at most 1 category or tag.

---
category: Foo
tag: Bar

In the above example, we specify a category named Foo and a tag named Bar.

Even if we write as follows:

---
category: Foo Bar
tag: Bar, Baz

we'll get a tag named Foo Bar and a category named Bar, Baz.

(2) Use string with plural key

Use plural key (such as categories or tags) and a space-separated string value.

---
categories: Foo Bar
tags: Bar Baz

In the above example, we get 2 categories (Foo and Bar) and 2 tags (Bar and Baz).

We cannot specify name which contains a space character. If you wants, use the next pattern "plural + string array".

(3) Use an array of string with plural key

Use plural key (such as categories and `tags) and an array of string value.

---
categories:
- Foo Bar
- AA,BBB
tags:
- Bar
- Baz, AAA

In the above example, we get 2 categories (Foo Bar and AA,BBB) and 2 tags (Bar Baz and AAA).

We can specify following, but in this case, we cannot specify name which contains a comma character.

---
categories: [Foo Bar, AABBB]
tags: [Bar Baz, AAA]

Read Jekyll's source code

To understand this complecated specification, reading the source code would be helpful.

See Hash#pluralized_array method in lib/jekyll/core_ext.rb.

This method is called like self.data.pluralized_array("tag", "tags").

class Hash

  # Read array from the supplied hash favouring the singular key
  # and then the plural key, and handling any nil entries.
  #   +hash+ the hash to read from
  #   +singular_key+ the singular key
  #   +plural_key+ the singular key
  #
  # Returns an array
  def pluralized_array(singular_key, plural_key)
    hash = self
    if hash.has_key?(singular_key)
      array = [hash[singular_key]] if hash[singular_key]
    elsif hash.has_key?(plural_key)
      case hash[plural_key]
      when String
        array = hash[plural_key].split
      when Array
        array = hash[plural_key].compact
      end
    end
    array || []
  end