I hear these terms used seemingly interchangeably, but is there a difference? Someone who plays loose is supposedly someone who bets often and likes to see many flops, so what does it mean to be an aggressive player?
The terms loose and tight are opposites and refer to someone's proclivity to fold or not fold, whether that's preflop or after the flop (or any street). A loose player will be staying in to see lots of flops and be most likely to be in a hand until showdown. A tight player is more selective of the hands they play and are usually only making it to showdown with good hands.
The terms aggressive and passive are used to describe someone's tendency to either bet/raise or to just be checking and calling. An aggressive player is always looking for spots to bet and feels comfortable re-raising in many situations, whereas a passive player is more often seen checking or calling. Aggressive players are trying to build big pots, give other players bad odds, and throwing in bluffs. Passive players are just trying to make it to showdown without creating any extra risk.
This creates four very general type players: loose-passive (usually pretty fishy), loose-aggressive (many good players have this style but so do some "maniacs"), tight-aggressive ("solid" players--can't lose too much with this style), and tight-passive (only play good hands but don't win much with them).
Now, these categories are pretty broad and don't exactly apply to everybody. For example, there are some people who are loose-passive preflop but then aggressive postflop; maybe they are very confident in their postflop skills so they elect to see as many cheap flops as they can to be able to use that postflop aggression skill.
As to your question about the terms being used interchangeably, there's a lot of times that people conflate the terms. After all, when you start noticing that somebody is betting a lot (aggressive), it might be natural to think they're also loose, but it might be only because you notice all the hands they are in. Passive players don't get noticed a lot because they're not instigating action, so they're often labeled as tight also.
Yes it is different. Loose means that you play with a lot of starting hand. The opposite is tight, the old way of playing poker, in which you play only with good/premium cards.
Playing loose, most the time don't give you a very good hand, so you have to play aggressive trying to take the pot with high raise.
If you play tight passive, you wait for good card instead of trying a lot of bluff.
Rare to see, but someone plays loose passive or tight aggressive...