There are 3 degrees of difficulty in catching Pokemons in Pokemon Go game:
1) green – simple
2) Orange – complex
3) Red – a very complex
The degree of difficulty Pokémon shows when you catching them and throw pokeball. By clicking pokeball near Pokemon is formed tapered ring its color shows difficulty level of catching Pokémon. The strong pokemon is orange or red.
Often orange and red Pokémon difficult to keep in pokeball so they can jump out and run away. To happen this much less there is special item in the game, berry Razz Berries.
This berry you get the first time on 8 level your pokemon. After that berries will appear in pokestop, with pokeballs. Or can buy them in the shop for game currency.
Use Razz Berries is very simple: at the time of capture pokemon before throw pokeball, click right bottom in the corner on bottom with backpack and there is a berry Razz Berries. It help Pokemon to not jump out from pokeball.