Tramadol is indeed an opioid, it's primary action is on opiate receptors and serotonin norepinephrine reuptake inhibition (SNRI). Tramadol, like codeine, is a weak opiate but when metabolized by the cyp2d6 enzyme it becomes O-desmethyltramadol. If I remember correctly O-DesMT is a great magnitude stronger than morphine, so just like codeine it is almost useless until metabolized into its major metabolite.
Some people are poor metabolizers of tramadol and codeine because of variations in the cyp2d6 enzyme and thus don't get very good pain killing effects from either of these drugs. So if you have tried codeine and didn't get very much effect from it you may be a poor cyp2d6 metabolizer and will probably need a different opiate to effectively treat your pain.
Personally I think tramadol is a great opioid, long half life and duration of effects so it lasts practically all day off one dose. Just be weary this drug makes you more susceptible to seizures especially at doses over 400mg.