My! You have a lot of questions!
What is the difference between white and black magick?
All magick is colorless, although some will say it is intent that colors it, I argue that it is a style and the will (for free will or not for free will). Much like our personalities, some of us are more aggressive and others passive. Both can heal, protect, and do everything else magick is capable of.
White magick claims to harm no one. But in my opinion there is never a guarantee that magick will not backfire or cause an outcome that you didn't factor in. To cast pure white magick, one must be pure of mind, and that is hard to come by. There are a lot of people who claim to be white and are not. But those who do follow the white path truly are pretty much "holy" in the modern context of the word. White magick is more of a passive approach that does not go against free will in any way. They will reflect, perform "justice" spells, bless, break spells, protection through shields, banishing through prayer and cleansing, calling to spirits for aid, etc
Black magick is a more aggressive approach. It's for the person that will get something done regardless of what needs to be done. This may be bending someone's will, binding (yes my fluffies "binding" is black!), retaliation, summoning, etc.
While most refer to black as "harming" the real line between these two is that white is pro free will and black doesn't really care. Both sides have their own beliefs about what is wrong or right and both are careful with how they choose to handle a situation. To be a black magick practioner isn't to be evil. Black magick becomes evil when the witch knows what they are doing is wrong in their heart and does it anyway because they just don't care about who it may hurt or that it may even hurt themselves. This is very dangerous and remorseless behavior.
When you get to know the elements you will notice that each has a dark and light side to them. These are very similar to the dark and light sides of magick.
There is also grey, which is inbetween the two. Grey believes in balance within themselves and within nature. Accepting the light and shadow side of one's self. In nature there is both dark and light aspects, and reflecting nature's ways, grey finds the middle and uses their instincts to decide the appropriate course of action for the given situation. Grey is very much about balance despite the slander that they do whatever they want to and call themselves "grey" to remove themselves from being seen as "black". (Yep, heard that one a good many times)
There is danger in every magick. Even healing spells, if you're not careful, you can absorb the ill energy and make yourself sick. The key to not getting hurt is knowledge. Learn everything about a particular magick and the type of spell you are casting before performing it.
Yes there are such things as summoning spells but they are for advanced magick users. You need to be strong enough, and knowledgeable enough to be capable of handling yourself if the situation gets out of hand. Again, knowledge is essential, not only in the magick and ritual you are using, but the being you are summoning. Exhaust your resources before attempting them. And become experienced in practicing magick. Summoning spells can be dangerous if you do not perform them properly or summon something stronger than you can handle. People perform them in order to ask or demand that the summoned performs a certain desire of theirs, or to gain knowledge from the being through communication.