Spellcraft is used whenever your knowledge and skill of the technical art of casting a spell or crafting a magic item comes into question. This skill is also used to identify the properties of magic items in your possession through the use of spells such as detect magic and identify.

Can you take 10 on a Spellcraft check?

Normally, as long as you are not in a situation of danger, such as combat, or being intimidated, or under stress (like climbing a wall), or being distracted by a traveling bard, you can Take-10 on your Spellcraft checks.

How does skill Focus work?

The description of Skill Focus says :You get a +3 bonus on all checks involving the chosen skill. If you have 10 or more ranks in that skill, this bonus increases to +6.