Here is an overview of the game, via the official Steam page: 

About

Assemble your team of Earth’s Mightiest Heroes, embrace your powers, and live your Super Hero dreams.

Marvel’s Avengers is an epic, third-person, action-adventure game that combines an original, cinematic story with single-player and co-operative gameplay*. Assemble into a team of up to four players online, master extraordinary abilities, customize a growing roster of Heroes, and defend the Earth from escalating threats.

Play an Original Avengers Story

Marvel’s Avengers begins at A-Day, where Captain America, Iron Man, the Hulk, Black Widow, and Thor are unveiling a new, hi-tech Avengers Headquarters in San Francisco. The celebration turns deadly when a mysterious enemy causes a catastrophic accident which results in massive devastation. Blamed for the tragedy, the Avengers disband. Five years later, with all Super Heroes outlawed and the world in peril, a sprawling adventure ignites when a determined young woman named Kamala Khan sets out to reassemble and rebuild the Avengers to stop the unchecked power of the secretive new force known as AIM.

Marvel’s Avengers continues the epic journey with new Heroes and new narrative delivered on an ongoing basis, for the definitive Avengers gaming experience.

Embrace Your Powers

• Progress through the single player campaign to rebuild your Hero roster and restore their powers; then continue to battle AIM solo or online with friends.
• Unlock powerful skills and new gear for each of Earth’s Mightiest Heroes to suit your playstyle.
• Customize these iconic Heroes with classic, fan-favorite, and all-new original outfits inspired by over 80 years of Marvel history.
• Flexible combat system, dynamically scaling difficulty, plus customizable gear and skills to support unique playstyles for each Hero.

Assemble Online

• Using a reclaimed helicarrier as a base of operations, players launch Hero Missions (single-player) and Warzone Missions (single-player or co-op) in hotspots around the globe.
• Up to four players assemble online to defend the Earth from escalating threats that only the Avengers can overcome.
• The narrative will unfold over multiple years, with every new Super Hero and region delivered at no additional cost.

*One-time online access required for single-player campaign; online access required for multiplayer and to download post-launch content. Free Square Enix Members account required. Beta release date(s), platform and region availability to be announced.

See the launch trailer below.