けど is one of the common conjunction particles in Japanese. These are particles that function by connecting the (A) and (B) parts of a sentence. けど works very similarly to が, in that it highlights ‘while thinking about (A), (B)’. This means that けど will often come across as ‘but’ in English.

• 毎日（まいにち）泳（およ）ぐけど、今日（きょう）は泳（およ）がない。
  I swim everyday, but I will not swim today.
• 北海道（ほっかいどう）は寒（さむい）いけど、綺麗（きれい）。
  Hokkaido is cold, but pretty.
• 田舎（いなか）は静（しず）かだけど、不便（ふべん）だ。
  The countryside is quiet, but inconvenient.
• これはステーキだけど、冷（つめ）たい。
  This is a steak, but it is cold.

As can be seen from these examples, just like many other grammatical constructions, だ will be required when using けど after a な-Adjective, or a noun.
けど is just a modern (casual) version of けれども. Please see below for a list of the most casual, to the most formal variations.
Casual - けど - けども - けれど - けれども - が - Formal

• 私（わたし）は肉（にく）を食（た）べるけども、野菜（やさい）も好（す）き。
  I eat meat, but I also like vegetables.
• 私（わたし）は水（みず）が怖（こわ）いけれど、泳（およ）ぐのが好（す）きです。
  I am afraid of the water, but I like to swim.
• お酒（さけ）は飲（の）むけれども、ワインは飲（の）まないです。
  I drink alcoholic beverages, but I don't drink wine.
• 沖縄（おきなわ）は綺麗（きれい）ですが、暑（あつ）いです。
  Okinawa is beautiful, but hot.