Hashtags are widely used on Instagram. Hashtags are words or phrases marked with the # symbol and are used to enable collecting all content with that tag in one place.
With public Instagram accounts, anyone searching for a hashtag used in a photo caption will be able to see that image. This is a great way to get specific photos and brand accounts a lot more attention. Like on Twitter and Facebook, users should be smart with hashtags: Do not overuse them, and only use words that are actually relevant to the photo being tagged.
Some brands overuse hashtags with the goal of getting more likes and followers. This is not going to get the followers they want and will likely attract spammy comments.