When I create a new token, how can I differentiate it from another token? Can various NFT tokens exist with the same ticker?
Tokens are uniquely differentiated by their Policy ID and Asset Name, which are combined to make a unique Asset ID. The Policy ID is a hash of the monetary policy script that was used to mint the token, and the Asset Name identifies the token under that Policy ID. So long as you just mint one token with the a particular asset name under that policy, the NFT will be unique.
So, the ticker (its Asset Name or the "Ticker" field its minting metadata) is not unique by itself, but the Asset ID or its Policy ID will distinguish it from other tokens with the same ticker.