I'll try and give my best description on competitive battling.
Competitive battling is using a specific team that is made to win. No gimmicks or anything like that. Your team is built to best exploit your strengths. To do this, you have to raise a good team. This involves planning your team out, then building it. Building a team focuses on pokemon that combo well, as well as using useful pokemon in general. Be aware of EV'ing as well, because EV'ing is used in all competitive battling. Effort Values, of EV's for short, are used to boost a pokemon's stats to greater levels.
The most competitive pokemon fall into he overused tier. Tiers are considered the rankings for pokemon. If a pokemon is good, it's in the overused (OU) tier, or standard tier. Scizor and Tyranitar are examples. If it isn't very good, it fall into the underused tier, or UU for short. Venasuar and Feraligatr fall here. If a pokemon is a gimmick, or extremely weak, then it falls in the neverused tier, or NU, for short. Farfetched and Delibird, for example. If a pokemon is deemed too powerful for standard play, then it goes to Ubers. Pokemon like Kyogre and Mewtwo go here.
The biggest tournament is the Video Game Competition, or VGC for short. This happens once a year, where you use your team to compete against other teams. There aren't very manyother organized tournaments, but you can battle over wi-fi, or pokemon online. Pokemon Online is a server where you can battle pokemon without raising them up in the games.
Sandstorm teams are teams that focus on the weather effect, sandstorm. Hippowdon and Tyranitar both summon an indefinite sandstorm, which other pokemon benefit from. Doryuuzu, for example, is pokemon who's speed doubles in a sandstorm. Garchomp's evasion is raised in a sand storm. Rock types get a Sp. Def boost in a sandstorm. These are just a few examples. There are other weather teams as well, including rain and hail. But right now, sandstorm is dominating.
Blissey and Salamence are OU/Uber pokemon. They are very powerful, and such, are used often. Blissey is used because of it's massive HP. Salamence is an offensive beast. These pokemon can be used in both OU and Ubers, although Salamence as banned from OU. These pokemon do their jobs very well. As per Shandera, it is a new japanese pokemon. It's secial attack is very high, so it is a good special attacker. On top of that, it also has shadow tag. This means that the opponent can't switch out their pokemon while Shandera is in play. This means Shandera can get set up. People like to find good combinations within pokemon. That's the deal with those guys.
I hope this helped. It's as much as I know about competitive battling. You should check out Smogon.com or serebii.net. They have excellent guides on competitive battling.