Tramadol has opioid activity, so it's an opioid painkiller like codeine or morphine. However, it's not a hugely strong opioid and this activity appears to come from one of it's metabolites, o-desmethyltramadol.
Tramadol is also an SNRI (serotonin and norepinephrine reuptake inhibitor) - the same class as venlafaxine, and similar to SSRIs. However, it isn't used in this way as it's opioid properties make tolerance and addiction a big problem. The SNRI activity can be quite stimulating as it increases serotonin and nor epinephrine levels in the brain.
As mentioned, don't take it with SSRIs or SNRIs as you risk serotonin syndrome. This is essentially a condition where there is excess serotonin in the brain. It can be seen of as a more extreme version of serotonin side-effects you get from taking a pill (or taking a SSRI/SNRI sometimes) - high body tempterature, pupil dilation, muscle tension etc - only with serotonin syndrome you also have unstable blood pressure, even higher body temperature, fast heart rate, agitation and mycoclonus (twitching muscles). It's a medical emergency; people who think they had it for a bit then it went away likely just had severe serotonin side effects; actual serotonin syndrome needs medical treatment. There is a good article about it
here.
As for tramadol causing seizures - 400mg is the accepted safe amount, but people have seized off less, and some people get away with more. Just because you have got away with more once doesn't mean it is always safe for you to take over 400mg. Taking drugs that lower the seizure threshold Imake seizures more likely) are a bad idea with tramadol -
link - where as benzos and gabapentin can ben protective (but don't rely on this, you don't know what dose you'd need and it doesn't always work anyway)
There is some more info in the Tranadol section of the Other Drugs directory, found
here
edit: people's recreational response to tramadol varies hugely -some people get a lot from it, some hardly nothing, some find it speedy, some find it feels like any other opioid... etc