RuneScape runs completely through your web browser, the only download is a small plugin. Instead of basing the game off classes, it is based of skills. Examples of skills include: cooking, fishing, mining, smithing and craftsmanship. You have a combat level, but it is determined by 3 separate skills: strength, defense, and attack. Each are trained by fighting, but they level by selecting a different attack style (ie. if you attack defensive, the experience goes to your defense). This game is controlled only by mouse clicking.
RuneScape has undergone many changes throughout the years. Last we checked, the game had Non-PvP servers, PvP servers, and Bounty Hunter servers. Non-PvP servers have no player battles whatsoever. PvP servers allow players to attack each other anywhere except safe zones (typically towns). Bounty Hunter servers give you a ‘target player’ to kill, if you kill that player you will be rewarded. The buying/selling system sets a price range for goods; sellers can price their items anywhere within that range.
With a membership, you gain access to a HUGE amount of quests, skills, items, dungeons, and much more. The free areas of the game are the ‘big cities’, but they don’t offer as much as the member areas. As a free member, you will have over 30 quests, but as a member you have hundreds. Almost all RuneScape updates only involve the members, you seldom see an update for free users.
Members have access to more content, not a combat advantage. Free Users will not be at a disadvantage because Member Items will not work in Free User areas.
You may want to play through most of the free game first; if RuneScape still holds your interest then purchase a membership.