The map and objectives: towers, Roshan, runes

What on the map is worth gold and time, and what teams take risks for.

Buildings

Three tiers of towers on every lane and two towers by the Ancient. Destroying them gives the whole team gold and opens up the map.
Barracks behind the tier-three towers. When all enemy barracks fall, your creeps become mega creeps — almost always the end of the game.
Glyph of Fortification makes buildings invulnerable for a few seconds. Check whether the enemy has used it before you push.

Roshan is the strongest neutral monster. His pit moves with the time of day. Killing him gives the team the Aegis of the Immortal — an extra life for a limited time — and later other rewards.
Tormentors are stationary monsters that reflect damage. Killing one gives the team an Aghanim's Shard.
Prepare for Roshan in advance: vision around the pit, team strength, enemy key heroes dead or away.

Power runes in the river give a temporary boost (haste, double damage, invisibility, illusions and more).
Bounty runes give gold, wisdom runes give experience; they are worth fighting for, especially for supports.
Lotus pools restore health and mana, outposts give vision, and Twin Gates move you quickly to the other side of the map.
Details (spawn times, rewards) change from patch to patch — check the latest changes in the patch overview.
Patch overview