At a Restaurant

and tapping are but two of the words you should be familiar with
when entering a restaurant.

Now tip and tipping is
when you let somebody, e.g. the waitress, keep the change.
You can say that you tip the waiter or waitress, and during the proper circumstances
it's simply
a gratitude, a token of appreciation you might say.

Tap can be a faucet, or a
hole in a beer barrel, from which beer is tapped, drawn or poured.
Beer tapped from a barrel
is  often referred to as draft beer (AE) or draught-beer (Br).
Now, let's say there's a sign on
the wall facing you while you are standing at the bar,
counter, saying Special Brew of the
week  only £2.50 a pitcher.
The sign refers
to beer on tap, draft beer, which is tapped  into a pitcher and then
you can have
pitcher to pour from,serve from, on your own,
Let's say all four of you, if you are a party of four.
This way you don't have
to  ask the bartender, the tapster, the barman to fill  your glass.
Also, draft beer of course, is
generally  more inexpensive, cheaper, than bottled beer,
beer on
the bottle.

Tapping gently on or at a
door doesn't mean one is drawing beer in a gentle way at that door,
but rather it says
that one is lightly knocking.
Tapping and knocking are
sometimes synonymous, just as tap and knock are.

tip on the other hand, as mentioned previously,
could mean that money is given to a waiter or waitress
as a token of appreciation.
Well tip,
just  like many other words, can mean more than this.
Tip sometimes corresponds to Swedish "tipp/tippa".
As a
result, or should we say consequence, one can tip not only the waitress or waiter,
but also a truck or a wagon. Is that funny or what ?
If, for some reason, this
makes you confused, you must bear in mind that Swedish  has many, many words which are just as well very confusing to the beginner or non-native.
Principal parts of verbs are often confusing to someone having a language as a second language,
because verbs
act  differently in various languages. This is true of many words.
One can never ask for strict logic
when  dealing with a language.
 One often tends to ask
for  logic in a foreign language while studying.
in mind though,
there is hardly
much  logic in any language
other than
in  limited areas.
Moreover, the logic referred to, or
asked  for, frequently tend to be depending on
your  mother tongue, and even if there is quite  an organized, and strictly defined structure and grammar  in your own language, don't expect logics from  a foreign language.
You are only creating a
problem for yourself,
making things more complicated than necessary. 
Be flexible, look, listen and learn. Check for feed-back and response in various ways. Look for characteristics, and make up your own little library, trying all the time to find suitable references to each new word, and also if possible a suitable context, environment, with which  it should be stored, memorized. Words in most languages tend to live their own lives, just as we do.
Language, being reflections of humans and human endeavors, fortunately has better endurance than we have.

I hope you've found out  one or two things by now.