Each ingredient has four possible effects. From Alchemy levels 1-24, you can only see (and use) one of those four. 25-49, you get two, 50-74 you get three and 75 and up you can see all four. (When you hit level 100, you can make potions/poisons with a single ingredient.) As you gain levels, your potions get more and more powerful. Also, there are four different pieces of Alchemy equipment - having all four on hand make poisons and potions more usefull.
Try different combinations - you can make some really powerful and usefull stuff. Also, combining ingredients often results in a potion that sells for more than the ingredents seperately, you it can be a good money making source if you just want to get rid of stuff that doesn't have effects you are interested in.