Members will be able to add multimedia to tweets - including pictures and videos - without eating into the 140-characters-a-post limit.
The service is also changing the way it handles conversations between users.
Twitter co-founder and chief executive Jack Dorsey told the BBC his aim was to ensure that "when people tweet, it makes sense".
One analyst said the moves marked a "positive change", but added that they only addressed "one symptom" of Twitter's difficulty in increasing its audience.
Despite constant references to tweets in the news, over the past year Twitter has struggled to attract fresh users to its platform - a problem partly blamed on it being confusing.
"One of the biggest priorities for us this year is to really refine our product, to make it simpler," Mr Dorsey said.
"I think there's a story to be told about what Twitter's for, and to continue to strengthen why you would use Twitter."
Some of the details were reported by the Bloomberg news agency a fortnight ago, and were, for the most part, welcomed by users.
'Doesn't make sense'
The changes, as outlined by Twitter, will be:
- media attachments, such as photos and videos, will no longer count towards the character limit
- @names in reply to tweets will not be counted
- people will be able to retweet and quote-tweet themselves, enabling them to resurface any of their previous posts and add new commentary
In addition, any new tweet - ie one that isn't a reply to someone else's tweet - that starts with a username will now be seen by all of a person's followers.
That last change does away with one of Twitter's more baffling systems, in which posts beginning with a username would only be seen by a person's followers if they too were following the member mentioned at the start of the tweet.
To override the rule, people have been adding a full stop to their tweets, so that they read ".@username".
"It doesn't make sense to anyone," Mr Dorsey told the BBC. "And people have had to work around it. That just looks ugly, and it's confusing."
Time to adapt
Brian Blau, an analyst at the consultancy Gartner, said Twitter's problems in gaining new users would not be solved with these changes.
"The core problem is attracting new users and getting them to be loyal users over time," he explained.
"And we haven't seen anything from Twitter yet that leads me to believe that they're addressing that fundamental problem."
The changes will not be made until later this year.
This is to allow developers to integrate the new rules into their third-party apps and websites.