First note small and crossed out tied to second note with a curved line. Slides are straight lines I thought, and usually I’ve seen hammer ones marked with an H over it, so I’m not sure what this would be. I’m guessing that it’s a hammer on and the H I’m used to seeing is a simplified way of showing it, but I’m not sure.
This example is from Have You Ever Seen the Rain by CCR but I’ve seen the same notations in other tabs like Jolene.
It’s called a grace note. It’s not really notated in the rhythmic grid, but it usually is implied to play it just before the next note and often either lower in volume or hardly accentuated. It’s an ornament to the main (following) note and shouldn’t overshadow that main note in any way.
In real life, a lot of this is open for interpretation - use your musical intuition to phrase this properly. As often: context is king!
[Edited: removed my claim that it’s not a hammer-on… see further details in Gio’s reply below 
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Thanks, I had wondered that myself.
I appreciate this
+1 to everything @joergkutter said, except one important thing.
The grace note is usually played as either a slide or a hammer-on.
In this song it is absolutely a hammer-on.
The first note is played on the beat, but you hammer into the second note just as quick as you can, so there’s no rhythmic value to that first note.
Grace notes can have all different sorts of flavors.
It’s the weird thing you get when you try and write down how people organically play music.
I find that if I hit a grace note in music I haven’t heard before, I usually have to listen to the original to figure out how best to execute it.
In this specific case - it’s a hammer-on.
So, only one pluck?
Thanks for catching (and rectifying) my mistake, @Gio . I guess it makes most sense from a “technical” point of view to play it as a hammer-on; a double pluck would be cumbersome indeed.
On further thought, I guess I would probably have played it as a hammer on in any case😄
I mean, I really never know with grace notes.
It’s like trying to write an accent into text form… y’all.
It’s hard to know how to execute it.
I had to go find the recording and give it a listen. You can hear the quick little hammer-on in the recording. That’s really the only way.
When you’re sight reading something you’ve never heard before or can’t listen to, it’s trickier.
Absolutely!
But, now that I have you: you said you’d play the grace note on the beat and then (super-quickly) do the hammer-on to the main note. However, you could also play the main note on the beat and start the grace note a tiny bit before, no!?!
Again, up to context, interpretation, temperament!?!?
@Gio Would that be the same for this? It’s Pink Floyd and I can’t get that E G E D then back to C in time. Maybe a hammer on or pull off would be easier???
@Rob5589 - what song is it from?
I’d have to put my ears on it. Tab is just too unreliable!
EDIT -
It’s ‘Coming Back to Life’ right?
OK. Listening now.
Aiiight @Rob5589 - it’s a bad fingering/tab.
Right notes, but not how any bassist would ever play it.
You should play it with a ring or pinky finger slide 5th fret to 7th fret on the A string (D to E) then play the index on 5th fret of the D string (the G) and that’s plucked, then slide (again with pinky or ring) from the E back to D (so, 7th to 5th on the A string), then index on the C, and that’s plucked.
And yes - that D-to E (and then the E back to D) is a grace note.
Not a real rhythm, just a sound that is hard to translate into symbols.
Hope that’s clear!
The curved line is a slur and is commonly a slide or a hammer-on.
A straight line is a glissonado (or portamento). On bass this is also a slide.
The small crossed-out note is a grace note. It can be either crossed out or not; when it is, it’s like a short accent before the beat; when it’s not it’s more of a quick slide up or down on the beat.
(edit: oops missed all the other replies somehow before I added this 
)
Yep, that’s the song, thanks! @Gio 
If there’s no line through the note it’s an appoggiatura and the note takes actual time in the measure
If there’s a line through the note it’s an acciaccatura and it takes no time in the measure.
The appoggiatura steals from the note after, while the acciaccatura steals from the note before.
Yep. The biggest difference is if they are played on or before the beat, really.
