Refers to a seal used for closing envelopes, packaging, or official documents; often a decorative or security sticker.
封筒にシールを貼って閉じた。
I sealed the envelope with a sticker.
General term for decorative stickers, labels, or adhesive-backed illustrations.
ノートに可愛いシールをたくさん貼った。
I stuck lots of cute stickers on my notebook.
I stuck a cute sticker on my notebook.
I sealed the letter with a seal.
I neatly peeled off the price sticker from the product.
シール is a loanword for a sticker or adhesive label, which may be decorative, while 封 refers specifically to a closure meant to keep something shut.
seal set; set of stickers