Your team are stored in your cookies an such for your browser, so if you have Team A and Team B on Google Chrome, you will not have Teams A and B on Internet Explorer, you may have Teams B and C instead, provided you you actually inputted said teams. Also, as for tiers, they break up Pokemon into different groups, much like age groups at a school competition, but tiers are based on usage. If I Pokemon is used a lot it's obviously powerful or else no one would use it, and when Smogon (The main website in competetive battling, also the makers of the most common tier system) decides it is use too much that it is having a dominant effect on it's current Metagame is is usually moved to the next tier up. The tiers are Ubers, Overused (OU), Underuse (UU), Rarelyused (RU), Neverused (NU), and Little Cup (LC), along with Borderline (BL) and Borderline 2 (BL2). It seems very complicated at first but it really isn't. A list of tiers and the Pokemon able to be used in each one can be found here, and here is an introduction to Smogon's tiers, the most commonly used tier system, also the same that Pokemon Showdown uses.