Loanword from English 'junk', used for worthless items, clutter, or low-quality things. Common in compounds like ジャンクフード (junk food).
This room is full of junk.
I sold the broken computer as junk.
ジャンクフードばかり食べていると体に悪いよ。
Eating nothing but junk food is bad for your health.
I like rummaging through junk bins in Akihabara.
This camera is basically junk now.
ジャンク means 'junk' or 'garbage' (e.g. ジャンクフード), while ジャンキー refers to an addicted person or enthusiast; they sound alike but are unrelated in meaning.
Loanword from English 'junk'.