Hammer Ons:

It's most common to see the letter h representing a hammer on, located within the tablature between the original fret, and the hammered on fret (eg. 5h7). The concept of the hammer-on is fairly simple - playing a note, then, WITHOUT re-picking, playing another note on a higher fret on the same string. See the example:

Pull Offs:

The pull-off is a BASS technique that is, in a way, exactly the opposite of a hammer-on.
Similarly, the letter p is generally used to represent a pull off, also found in the same location within the tablature (eg. 9p7) Pull offs look very similar :

In the example above the open strings would be sounded by pulling off.

Slides:

Generally, a / symbol is used to notate a slide up, while a \ symbol is used to notate a slide down. It is also not uncommon to see the letter s used to notate a slide.

In this example you need to slide *up* from 1th to 5th fret then slide down from 5th to 1th fret.

Slaps (slapping and popping):

Slapping (or Thumbing) the bass is a technique used by many bands since at least the 1920s and came into popular use in the 1940s. Slap bass provides a strong downbeat when the string is plucked and a strong back beat when it slaps back onto the fingerboard of the bass. It creates a very percussive sound and adds a lot of drive that is particularly good for dance music.

On bass guitar it usually refers to a technique that consists of hitting the string with the thumb of the strumming hand near the base of the guitar's neck, often combined with snapping the strings usually with the index or middle finger of the same hand, (the latter more specifically called "popping", i.e. "slapping and popping"). Some bassists use other fingers of the strumming hand to achieve this sound, such as bassist Abraham Laboriel, Sr., who conversely uses his thumb to pop the strings, and his other four fingers to slap the strings.
Generally, the letter S is used, under or over the note, to notate a slap (or thumb).

Generally, the letter P is used, under or over the note, to notate a pop (or pluck).

Palm Mute:

The palm mute, also known as palm muting or chop, is a playing technique for the guitar or bass.
Palm mutes are executed by placing the side of the picking hand below the little finger across all of the strings very close to the bridge and then plucking the strings with the fingers while the damping is in effect. This produces a muted sound. The name is a slight misnomer, as the muting is usually performed by the side or heel of the hand.
Generally, the letters PM is used, over the note, to notate a palm mute.

Tie Note (or Legato):

In italian the word 'legato' (literally meaning "tied together") indicates that musical notes are played smoothly and connected. The note is audible but isn't attacked.
Generally, the letter L is used, instead the note, to notate a tie note (or legato).