How does magic work in 5e?

[Eladrinblade]

I've never played 5e or even looked at a book, but I've heard people talk about how the spellcasting is supposedly pretty different from 3.x, especially about how you have to concentrate on all lasting spells? Can someone give me the lowdown on how it all works?

[Skelechicken]

5e introduces "concentration spells" which are a lot of the buff spells and lasting spells.

A spellcaster can only have one concentration spell active at a time, and concentration can be broken by damaging the spellcaster or sufficiently distracting them. A spellcaster who takes damage while concentrating must pass a Constitution check with a DC of either 10 or damage taken, whichever is higher.

Being knocked unconscious automatically breaks concentration.

You can cast a non-concentration spell as an action and still maintain concentration, so a bard can use vicious mockery on a new target while maintaining Tasha's Hideous Laughter on a different target.

In addition to concentration some spells can be cast as rituals, allowing them to be cast without consuming a spell slot but adding 10 minutes to the casting time.

[TripleD]

There are three main differences:

1. Cantrips. Basically "level 0" spells that you can cast an infinite amount of times per day. Their power scales automatically with character (not class) level

2. Lower spells in higher spell slots. Level 1 and higher spells do not automatically become stronger with class level. Rather, you have to cast the lower level spell with a higher level spell slot to power it up.

3. Concentration. Some spells are tagged as "concentration". These spells provide a continuous effect, but you may only have one of them active at any given time. For example, a character can cast "Shield", but if they go to cast "Fly", shield automatically ends, since both spells require concentration.

One other change, stat bonuses do not give bonus spells.

[furby076]

Honestly, your best bet is to read the rules. Its not too much different than 5e. Many (not all) long lasting spells require concentration so you can only have 1 active at a time. While concentrating on that spell, you can still do other things (cast spells, attack, run, talk, etc). If you cast another concentration spell, then your first concentration spell dissipates.

[Lord Il Palazzo]

How spells are prepared and how they are (or aren't) used up is also pretty different.

In 3.x, a wizard might have 4 level one spell slots and 2 level two spell slots for the day. With that, he could say that he prepares Magic Missile, Mage Armor, Color Spray and Grease for his level one spells and Scorching Ray and Levitate for his level two spells. Once he cast the one Magic Missile he prepared, he wouldn't be able to cast Magic Missile for the rest of the day until he prepared spells again.

In 5e, the same wizard also might have 4 level one spell slots and 2 level 2 spell slots for the day, but he also has a separate number of spells prepared for the day equal to his level plus his intelligence modifier. (The number of spell slots a character has no longer means the number of spells he can prepare.) If he's level 3 with 16 intelligence, he would still be able to prepare 6 spells, but he wouldn't be locked into 4 first and 2 second level ones. He could prepare three and three, four and two, two and four or any other mix of spells from his spellbook. When the wizard actually goes to cast a spell, he spends on of his spell slots and picks the spell he wants to cast (usually the level of the slot he's using, but like TripleD said, you can cast a lower level spell with a higher level slot and sometimes get a bigger effect). Casting a spell uses up the slot you spent, but doesn't remove that spell as an option for the rest of the day. A 5e wizard could prepare all the same spells as the 3.5 wizard I listed above, and then use all his slots to cast Magic Missile all day.

I used a wizard as my example, but the same mechanic works for other classes too. Casting a spell never removes if from you list of options for the rest of the day as long as you have another slot to spend. Some classes will just know a set number of spells that they can cast with their slots instead of preparing a certain list each day, but that's pretty much the same mechanic 3.5 sorcerers and bards except that their spells known aren't broken down by levels. (A level 3 sorcerer knows four spells of levels one and two, but the number of each isn't fixed, only the total.)

Edit:

Honestly, your best bet is to read the rules. Its not too much different than 5e. Many (not all) long lasting spells require concentration so you can only have 1 active at a time. While concentrating on that spell, you can still do other things (cast spells, attack, run, talk, etc). If you cast another concentration spell, then your first concentration spell dissipates.Giving the rules a read isn't a bad idea. Wizards released a Basic Rules PDF for free (link here (http://media.wizards.com/2014/downloads/dnd/playerdndbasicrules_v0.2.pdf)) that has cleric, fighter, rogue and wizard in it along with all the actual rules you need to play a basic game. If you check the PDF, the spellcasting mechanic is explained for clerics on pg. 22, for wizards on pg. 30 (basically the same as clerics but with a spellbook) and other spellcasting rules start on pg. 78 (with concentration specifically on pg. 79).

Edit 2:
Heh, sorry I kind of cut you off with my edit KorvinStarbast. Furby posted while I was typing mine and inspired me to link the Basic Rules.

[KorvinStarmast]

I've never played 5e or even looked at a book, but I've heard people talk about how the spellcasting is supposedly pretty different from 3.x, especially about how you have to concentrate on all lasting spells? Can someone give me the lowdown on how it all works?
1. Go to WoTC home page, and get this set of basic rules for free. (http://media.wizards.com/2016/downloads/DND/PlayerBasicRulesV03.pdf)
2. Read them.
3. Once you've taken a look at the magic description, come back with questions.

Magic works. Concentration makes it harder to stack a load of buffs.
Wizards are less Vancian, but still need books.