[2020] whats the difference between top and bottom lane?

Hi!

The main difference is the neutral objectives that the lane is closest to. Top lane is closest to the Rift Herald and bottom lane is closest to the Dragon.

The dragon is generally more valuable and easier to take early on in the game, so teams tend to position a Marksmen (a champion that is weak until later on in the game) and a Support (a champion that is a bit stronger and mostly exists to protect the Marksman, with a few exceptions) in the bottom lane.

This lets each team have four people able to take the dragon early game.

Because most of the focus early game tends to be around the Dragon area (again, it's easier to take and, in general, more valuable), top lane doesn't tend to get much attention. Champions that go top lane need to be fairly self-sufficient and strong right from the get-go, so these champions tend to be bruisers (who are strong melee range fingers), tanks or occasionally ranged champions (which can beat up bruisers and tanks). These champions also tend to be able to utilize the teleport summoner spell well; teams will usually call their top laner into a fight on the bottom side of the map using teleport.

Now, you might wonder, if a team intends to forego an early dragon and instead focus on surviving the early game, why they wouldn't instead send their duo to top lane, where there's only one person in it to oppose them. The answer is that.. well, they used to do that. It was a popular strategy a long time ago to send your duo to opposing sides of the map and take the first tower while your top laner walked with your jungler soaking up experience and gold - this was the era of the so-called 2v0. This no longer works because the top (and mid) lane tower takes significantly less damage from champions before 5 minutes, so the team that swaps their duo to top lane will lose a lot of resources.