Claircognizance

This is when you just know things.  The ideas, thoughts, and concepts pop into your head and if you're not paying attention, you can mistake them as your own thoughts.  Journaling, whether by paper/pencil, typing, or by voice or video is a really good way to stay familiar with the sound of your own thoughts, as is the modern practice of mindfulness.  This can help you learn to differentiate between a claircognizant thought and a thought that is your own.

Clairsentience

This is when you feel things that are not physically there.  I've recently seen people refer to "Clairtangency" and "Clairempathy" as separate clairs, but I would have lumped them in together under this clair.  Sentience is perception, so whether you're perceiving an emotion or a physical sensation or you just have a feeling about someone or something, I would call all of these different types of clairsentience.

Clairvoyance

This is when you see things that are not physically there.  It should feel similar to when you are having a daydream, and just like when you have a daydream, you may not be paying much attention to your physical surroundings while you are having a clairvoyant experience, so please use this carefully.  If you are having difficulty distinguishing between what is a clairvoyant experience and what is reality, please make sure you check in with a mental health professional.  You will be much less able to help people with your gift if you are yourself confused about what your ability is showing you, and a mental health professional can help you rule out mental illness as a potential concern.

Clairaudience

This is when you hear things that are not physically there.  This should not sound like an actual voice.  This should be similar to claircognizance in that the thoughts/sentences/words pop into your mind, but these statements may come with the sensation of a voice, as if you remember what someone sounded like even though you didn't actually hear them.  If you are hearing actual voices, please check in with a properly licensed mental health professional.  Clairaudience can also be hearing music play in your head, and this may sound like the actual song just like when you have a song stuck in your head and playing on loop.

Clairgustance

This is when you taste things that are not physically there.  In the context of mediumship, this might come through when a crossed-over loved one is explaining something about cooking or food.  In personal practice, your spirit team might use a specific taste to give you a warning about a person, situation, or place.

Clairsalience

This is when you smell things that are not physically there.  In the context of mediumship, this might come through as the scent of a perfume or incense that a crossed-over loved one liked to wear frequently.  In personal practice, your spirit team might use a specific smell to give you a warning about a person, situation, or place.

Timeline work is just like any other psychic skill in that it can be developed over time and with practice.  Some people will find that they have a natural talent in it, just like some people find that they are naturally talented at running or writing or memorizing things.  Meanwhile, some people will find that they have a lot of difficulty developing the ability to view timelines.

Viewing the past and the viewing the future are different in that looking into the past can't change what happened.  However, looking into the future will change the future, because whatever you are seeing is based only on what is true in your present moment, with the information you have available to you in that present moment.  After you have viewed the potential future, you now have that information available to you as well, and having that information automatically influences your decisions as you go forward - instantly, the future has changed.

There are very, very, very few future events that cannot be changed.  Making accurate predictions is almost impossible because of this.  Whenever you are given a vision of the future, always ask yourself "why"?  I have found that there are two reasons:  One, so that I can brace for impact and soften the blow; Two, so that I can influence the actions and choices of those around me and change the future outcome entirely.

Being given a warning about a potential future is far more common than being given a vision of a future that actually happens.

Astral travel is the ability to send your spirit/soul out of your body and travel to a different location, whether somewhere on the earth, or out among the stars.  Remote viewing is the ability to extent your clairvoyance so that you can see things that are happening elsewhere on the earth, or anywhere among the stars.

These abilities can also be extended to exploring timelines, parallel dimensions, and connecting with beings from alternate realities.

I don't personally recommend taking these abilities to that far an extent, because I believe strongly in the need and power of the present moment of the present reality.  However, it is possible, and may be beneficial under some circumstances.