Upon playing Crystal Core, all friendly minions in the player's hand, deck and battlefield get set to 5/5 stats, and any friendly minion played or summoned thereafter will be put into play as a 5/5. However, minions can still take damage, get buffed and activate their triggered effects, Battlecries and Combos like normal; they will simply start out with 5/5 stats by default.
All Jade Golems summoned after playing Crystal Core will be 5/5 minions.
Taking control of an enemy minion after playing Crystal Core, such as using Sylvanas Windrunner's Deathrattle or a randomly generated Potion of Madness, will cause that minion's stats to immediately be changed to 5/5.
Silencing a minion affected by Crystal Core will remove all enchantments and card text except the Crystal Core enchantment, meaning that the minion's stats will remain 5/5.