Hi DixieChristian,
As Matt noted, it varies and really does depend on what the developer is willing to let you do with the software while trying it out.
Software is generally divided into three offerings.
Freeware - Just what it says. The author makes the software he/she/they wrote available for free. Sometimes, but not very often, with a screen that asks if you use the software that you make a voluntary contribution or pledge a donation to a charity they are backing.
Shareware - Try before you buy. Usually software written by an individual or small company. It's almost always crippled in some way that allows you see what it can do, but keeps you from using certain features until you pay for it. Other times it's fully functional, but for a limited time. Others, like Pacifist, shows you a 15 second nag screen every time you launch it, but has no time limit and is not crippled.
Commercial - Adobe, Apple, Quark, etc. Pretty much anything you can find on a retail shelf. But there's also plenty of commercial products that can only be purchased online. Generally, you have to buy it sight unseen, going only by what the box or web site says the product is for. Not that you can't find trial software for commercial software. Adobe, Quark, Extensis and many others have fully functional software that will run for 30 days. In the past, Adobe's software used to let you do everything but print or save files. The latest trial software they have for their products lets you do it all. Again, it all depends on what the developer will let you do with trial software you need to pay for at some point.