Parler is basically a direct clone of twitter's basic features. They use different names but have features similar to that of Twitter's tweet/like/retweet.
Gab's current iteration is a mastodon fork (although iirc they don't participate in the fediverse via activitypub). Gab is similar to Twitter like Parler but is less of a direct clone, offering other features Twitter doesn't have.
Gab has been along for longer and seems to be more popular. Both market to the (mostly American) far right, with their shtick being basically "you can say anything that is not directly illegal in the US without fear of being banned or other moderation". Gab seems to have been more competent in coming back up after bans from their hosting provider and other partners.
Gab's current iteration is a mastodon fork (although iirc they don't participate in the fediverse via activitypub). Gab is similar to Twitter like Parler but is less of a direct clone, offering other features Twitter doesn't have.
Gab has been along for longer and seems to be more popular. Both market to the (mostly American) far right, with their shtick being basically "you can say anything that is not directly illegal in the US without fear of being banned or other moderation". Gab seems to have been more competent in coming back up after bans from their hosting provider and other partners.