I was never very good at understanding gambling odds until I had to run a book for our annual golf trip last year (and had to learn by being thrown in the deep end!). Everything you need to know about odds/gambling/book making/probability can be found in these three articles:
Requires a little bit of study but it’s worth it!
However, I still find fractional odds confusing and too difficult to process in my head (especially when being discussed in a pub!), so I wrote a very trivial mobile app that lists common fractional odds and their decimal equivalent (and associated probability). So if someone drunkenly offers you odds of 11/8 on something, you can do a quick lookup to see that 13/8 is 2.38 in decimal odds (i.e. you get back your money multiplied by 2.38), and also that 13/8 has an implied probability of 42.02% (i.e. the chance of this happening). I hope this saves me a fortune
You can browse to it on your phone here (and make a shortcut to it from your phones desktop for quick pub access). If you have an Android phone you can also download the app to your phones sd card from here and install it directly using an Android App Installer (see this article for instructions).