Hey guys I’m thinking about getting into brewing and was wondering. Sometimes I see informational videos detailing that the guy just uses a yeast packet and dumps it right into the fermenting jar. As of late I’ve been dabbling in more complex videos and more specific ones at that and noticed that a lot of people. have beakers full of yeast. I assume they’re doing this to grow the yeast, but do I need to cultivate it or whatever it maybe before I use it in my brew or can I just dump the packet of yeast in there. Thanks for the help guys. I’m just trying to understand this subtle science. 
Building up a yeast starter is always a good idea to ensure you have good healthy yeast. People have less problems with dried yeast that stores refrigerated for many years, but with liquid cultures, the yeast only has a life of approximately 6-9 months from date of manufacture before it’s pretty much dead. Nowadays the yeast companies will put a date of manufacture to help you know the life better. You can also use a yeast calculator to determine exactly how many packs of yeast or size yeast starter to make for any batch at:
http://www.mrmalty.com/calc/calc.html
The above calculator is known to be a bit conservative but it will help you get into the right ballpark.
If/when making yeast starters, use DME and aim to make a specific gravity of about 1.040. This will grow the yeast without stressing it with very high or very low gravity.
Learn more online everywhere. Sorry I don’t have any specific links. Starters are easy to make and very helpful. Not required, but nice to do, for liquid cultures anyway.
With dried yeast, you can and should safely skip a starter because it is so reliable. It is for that reason that many people including myself are trying to find and use good dried yeasts as much as possible, it’s just so much easier.
Hey thats a really good resource. Thanks for the information and I’m gone save that link.
There’s a better and easier way to make starters that was put forth on this forum. It’s all I’ve been doing for the last 2 years. I don’t use a yeast calculator and I honestly don’t know where my stir plate is any more! Take a look at this…Old Dog…New Tricks | Experimental Brewing
Plus me
+2 no more stirplate…
What’s a stirplate? 
Still use the stirplate.
Running the stirplate keeps yeast in suspension for more even distribution when harvesting. I deliberately overbuild starters to harvest yeast, then pour off a portion of the starter to save.
I’m in JTs camp. Stirplate, overbuild, keep some for later.
Though oxygenation is only needed at the outset, a stir plate will keep yeast in suspension – maximum contact between yeast and medium – and continually drive off CO2, both potential benefits. Your call. Occasional jostling of the jug might be just as good.
When you make a starter in an Erlenmeyer flask, how do you close the top? I use a foam stopper, but I’ve heard of others using an airlock.
Loosely cover with a piece of foil.
I put a piece of foil on it when I cold crash.