Not all are scams, most are inducements to play but the important aspect is the fact that like all gambling concerns, they are there because they can afford to do so, from their losers and if you play any game against ther odds, you will lose, and allow them to offer others this "come on" from your money.
Lasty, not all poker sites have any one playing against others, they almost all, have computers to make up the numbers on games like Texas hold em, if you want to find out which ones, get the magazine "Card Player", and you will soon find many others "Online Card Players" is another, there are about seven magazines available and many list the sites to avoid, as well as the good ones, see W.H.Smiths and ask them.
It is important that you only use those sites that are verified or approved, and audited, and if possible in your own country, and as such, covered by the gaming acts of that nation, many overseas sites are less than honest, and not covered by laws or even ever will be, and you need to know which ones, just to avoid the thieves out there, especially those who are only after your credit details and address.
( THEY CAN GET LOANS WITH THIS INFO AND STEAL YOUR IDENTITY AS WELL FANCY WAKING UP WITH A MORTGAGE YOU DID NOT APPLY FOR ? AND A HOUSE THAT HAS BEEN SOLD AS WELL, WHILE THE BILL FOR THE LOAN IS IN YOUR NAME ?) so only use a pre paid internet card for any payments, not your usual, and preferably one that pays by mastercard like "PRIME", "Maestro" is not a worldwide card, and will no longer be accepted by googlepay, from August, 2009.
ALSO IF YOU MUST PRE PAY, INTO A FUND OR ACCOUNT, BEFORE PLAYING, ONLY PAY INTO THIS ACCOUNT IF YOU ARE SURE THAT YOU CAN GET YOUR MONEY BACK, AND THAT IS NOT ALWAYS LIKELY FROM FOREIGN ACCOUNTS OR COMPANIES or as is often possible pay into this account ( even if you believe it is safe ), by paypal or some other service that does not pass your details on for any other parties usage.
But get the Magazines as a first stop, or log onto the ( real ), casinos websites, in England to see a few card rooms, with real games and try a few, if you are old enough, quite a few have competitions where entry charges can be as low as £5, and re buys are limited (up to one hour), they are a cheaper alternative to on line games and many, like the Birmingham casinos give these magazines free to all players, or members, ( these can cost about £35, a month to buy, and some also have six months of back issues they will pass on to you as well, the info in them comes from professional poker players and in Edgbaston, this means only one place, where the players are also the authors and winners of Vegas prize competitions, the very heart of the game in England ) well worth any entry fees to a game, ( none of which the casino keep, all of these are returned as prizes, and often the casino will also add to the overall; prize for nothing extra ), look particularly into the Edgbaston locations, for casinos, ( I work in one and we are restricted in our rights to advertise, hence I will not give the name, but look for "Portland rd", anyway,
Casino Gaming Inspector.... 35 years of boredom in this industry, ( as Robert Mitchum says, in his character, "I forgot how boring gambling can be, if it is not your money on the table" " The Unforgiven"