Once you hit lvl 18, you unlock the Strike Playlist. This shows up on the navigation menu in orbit, next to the Crucible option in the bottom right.
There are 4 playlists. One for lvl 18, 20, 22 and 24. All this is is a Fireteam of 3 like any other Strike, but you get put in to a random strike, REGARDLESS OF WHETHER YOU HAVE COMPLETED THE STRIKE OR NOT (so if you have yet to explore Mars, you may get thrown in to a Mars strike).
The lvl 18 strike gives you no marks, 20 gives you 2, 22 gives you 3, and 24 gives you 6.
These give you pathetic amounts of rep as well, ranging from 10 to 20.
At lvl 20, you unlock the daily heroic story and weekly heroic strike. The daily heroic resets, yep, daily. The weekly heroic resets every Tuesday.
Each have modifiers such as enemies do extra melee damage or enemies deal but are more vulnerable to certain types of damage like Solar for example.
The Daily heroic story mission gives you rep and marks, usually 5. Sometimes you can get rare material rewards, used to upgrade your rarer gear.
The weekly heroic not only requires you to be in a fireteam before starting it, but it gives you 3 marks, +150 rep and +3 Strange Coins (used to buy gear from the black market vendor that ONLY appears between every Friday and Sunday, with a minimum of 7 coins needed for each item he sells - and his gear is sweeeeeeeeet for sure - note that coins can be obtained via blue engrams as well as getting gold in public events, but they're not guaranteed).