Abilities are special effects some Pokmon have that can change gameplay, like drawing extra cards, healing, or affecting your opponents Pokmon
Most Birds have flying, making it even harder for your opponents to block them
Set boosters, by comparison, are said by head designer Mark Rosewater to have been designed specifically with the purpose of making packs more fun to open
These decks often include red, splash blue, deck wins, counter burn, UW weenie, UB fairies style agro control and various mono blue creature decks with things like Vedalken Shackles, Grand Architect and Opposition
"Pokmon TCG Pocket's Allegedly Plagiarized Cards Have Been Replaced"
There is also a link that will share the drop rates for each set and explain how the system for opening packs works on PTCG Live