Explore -- tab that showcases what are deemed "the most recently interesting photos" by Instagram; not just the photos with the most 'likes'/popularity contest, "interesting" is determined via a proprietary formula that takes multiple factors into consideration
Filters -- arguably the defining characteristic of Instagram; pre-loaded stylistic adjustments users can apply to their photos, producing various artistic effects; used to produce different 'looks' akin to physical filters used in conventional photography; current filters in v3.4.2 - Normal, Amaro, Mayfair, Rise, Hudson, Valencia, X-Pro, Sierra, Willow, Lo-Fi, Earlybird, Sutro, Toaster, Brannan, Inkwell, Walden, Hefe, Nashville, 1977, Kelvin
Follow/Follower/Followers/Following -- to follow (v) a user on Instagram is to choose to see their Instagram photos on your home screen; doing that makes you a follower (n) of that Instagram user, just as someone who chooses to see your photos is your follower (n); your followers number is how many users who have chosen to see your photos, your following number is how many users you have chosen to see their photos
Like -- (n) the option on photos that allows users to show their agreement with/approval of/positive response to that photo
(v) the act of selecting that option - symbolized with a heart icon (similar to liking a post on Tumblr) - to openly show your agreement with/approval of/positive response to the photo
News Feed -- tab in Instagram where likes, comments and new followers for both your account and accounts you follow are displayed; shows the activity among the users you follow; only comments shown are when someone you follow comments on a photo from someone you also follow
Photo Map -- feature that plots photos you've taken on a (Google) map, using the coordinates logged when you take a picture; shows where you have taken photos, allows for you to explore where others have taken pictures; goes beyond just recently taken photos, facilitating exploration of past photos; default setting for adding location/adding to Photo Map is off; can edit your map, no photos appear on map without user permission - user in control of what is on their map and when
Weekend Hashtag Project -- ongoing series run by Instagram; a new theme is chosen each week, announced on Friday and users are encouraged to take photos relating to the designated theme over the weekend, tagging those photos #WHPtheme (where theme is the chosen theme of the weekend)