Sonic the Hedgehog is Sega' most famous and popular mascot character, so it's no surprise that one of the Dreamcast's first major releases would be a new Sonic videogame. Sonic Adventure was a bold attempt to advance the Sonic franchise in much the same way that Nintendo had transformed and updated Super Mario Bros with the 3D action-adventure title Super Mario 64. To be honest, Sega's attempt was only a partial success - but those bits that did succeed were a lot of fun to play.
The reaction to Sonic Adventure back in 1998 was fairly divisive. Some players really enjoyed the game's attempts at a 3D environment and a more exploratory role for the player. Others felt that the game failed to capture the primary aim of a Sonic title; running very fast down a track to complete a level as quickly as possible. Certainly that sense of running through a level slightly too fast to feel one was in control was a key factor in previous Sonic games. The trick to enjoying Adventure seemed to be to acknowledge one wasn't playing a 'proper' Sonic game at all, but rather something entirely new.
The game is divided into two kinds of levels. The action levels are pretty much what players were likely expecting: typical speed-running Sonic gameplay in a 3D presentation - it wasn't a fully open world space, more like a corridor to run down, but it was energetic and fun and not too challenging. The adventure levels involved a lot more walking around a proper 3D environment undertaking a treasure hunt; sometimes they were fine, but occasionally these levels became interminable. The combination felt a little messy.
There was also a minigame attached utilising the Dreamcast's memory card, which had a small liquid crystal display that enabled it to be independently used as Tamagochi-style virtual pet. It was far from a necessary element of the game, but did add value.
Opinions really differ on Sonic Adventure. Some loved it, others hated it. I loved the bits I loved, and hated the ones I didn't.