There are three levels of play you can do.
1) Free-to-play: Don't spend a single dime on the game. You can play the game perfectly fine like this up to level 50. The biggest downside I could find was that you are restricted to 2 character slots per server. It's been a while since I played the game under this style, but it is doable, just more of a grind. I think you also do not have access to the Galactic Trade Network (auction house), but I could be mistaken. You are not able to equip anything that is purple in rarity, which is the highest power. You also earn experience at a lower rate than someone who is subscribed.
2) Preferred: Spend at least $5 on the game and you get to be preferred status. This allows you to access the GTN (if you weren't able to before), and also allows you to have 5 or 6 character slots per server. You have full access to the game, but still can not level past 50 or use purple items. You earn XP at the same rate as a F2P player
3) Subscribed: You are paying the monthly membership and have 100% access to the game. I believe when you subscribe they also give you access to the expansions that come out, meaning you can now level to 60. Every month they give you 500 Cartel coins which are the in game premium currency to buy things like upgrades for your ship, character, or specific suits of armour or weapons.
I stopped playing a while back when they switched from a skill tree, where you have more freedom to decide how your character progresses, to a more linear advancement making everyone more cookie-cutter with little variance.
The one thing I enjoyed the most about it is how Bioware does their RPG's, allowing you to interact with characters. When you are getting a quest, it isn't just a scroll that pops up saying "this is what you need to do." You go through a conversation with the NPC and they give you information as well as a bit of a back story. But you are interacting with them and your character will speak as well. Certain conversation choices will give you either light or dark side points, so you can have a light side sith or a dark side Jedi.
The instances are also interesting because there will be interaction points where the group must collectively decide what to do. The choices will be Light or Dark side oriented, and the majority will win. An example is in one where you board a Mandalorian cruiser and come to a control panel seeing a bunch of crew working in a hangar bay. Do you lock them in (Light side) or vent them into space (Dark side)?
The character quest line is always interesting (except for the Jedi Consular, I found), and there are plenty of side quests on each planet. So if you don't subscribed you can keep your level up to appropriate levels, you just have to grind through all of the quests that are available on a planet. The downside is, is that it can get quite tedious at times.