Tramadol's classed as an opioid analgesic - basically (as has already been mentioned) it's a drug for taking pain away that is similar in chemical structure and composition to other opium/morphine like drugs.
Here's what it says in the BNF (the book your GP looks in before prescribing most things):
TRAMADOL HYDROCHLORIDE
Additional information interactions (
Tramadol);
hepatic impairment;
renal impairment.
Indications
moderate to severe pain
Cautions
see
notes above; impaired consciousness; excessive bronchial secretions; not suitable as a substitute in opioid-dependent patients
General anaesthesia
Not recommended for analgesia during potentially light planes of general anaesthesia (possibly increased intra-operative recall reported)
Contra-indications
see
notes above; uncontrolled epilepsy; acute porphyria (
section 9.8.2)
Side-effects
see
notes above; also diarrhoea; fatigue; less commonly retching, gastritis, and flatulence; rarely anorexia, syncope, hypertension, bronchospasm, dyspnoea, wheezing, seizures, paraesthesia, and muscle weakness; blood disorders also reported
So, a possible side effect is muscle weakness, which might answer your first question, but this should be taken in context. Not all people who take tramadol will experience these side effects, just like any drug you take. If you read the cautions in any off the shelf medication, you'll be amazed at the amount of side-effects they mention!
As far as sperm count goes, I doubt it. It's not fully understood exactly how tramadol works or all it's possible action and side-effects. Maybe one day they've find out it gives you super-sperm? Or maybe fewer. I have read that it has been used to treat premature ejaculation, but I was looking at it more from a post operative pain controlling medication, so I didn't read anymore than that.
You haven't said why you want to know these things. Maybe talking to your GP might shed a bit of light on it?