I will compare them to Spanish ( Castillian/Castellano) 

Galician-Castillian

Probably the closest language to Castillian. I would say about 95% unsderstanding between native speakers when they are listening or reading. Grammar is very similar and is also very close to portuguese (actually it's a derivation of the old Galician-Portuguese).

Hola(cast)-Ola(gal.) ;

Hijo-fillo (keeps the ph from latin);

cuerda-corda(also has a different aproach to dipthongs) ;

llave-chave( "ll" is "ch") ;

ejército-exército ("j" is "x",from old latin, and old spanish ,see: texas,mexico,etc. Are pronounced "j" , hard english "h".)

Pronunciation is also very close , if you know spanish you can read galician because they use almost the same phonems.

Catalan-Castillian

It's also very close to Castillian but not as much as Galician, specially in pronunciation, some say it's a mixture between French and Spanish. Grammar is close i'd say about 85% .

Here are the major differences:

Like French they shorten articles:

La hora (cast) - L'hora (cat)

Words usually end with consonants instead of vowels in Castillian.

Bizcocho- Bescuit

Dorado-Daurat

They also keep sounds from latin that Castillian has changed:

Hijo-Fill( "F" transforms into "H" in cast. but every other latin language keeps the "F")

Ejército-Exèrcit( same as galician "j"-"x")

They also use different letters:

Ñ-Ny (España-Espanya)

/S/- Ç (Barça)

--And many other..

As you can see there are many similarities between all the languages that exist in Spain , but they are languages and not dialects because there are major differences.

I can't speak for the calatan but the spanish (castilian) pronunciation has to do more with its heritage from the euskera. It is a language whose writting macthes its pronunciation whilest catalan looks in this issue like any other language which has a slight diference in writting and in pronunciation giving it a more colourful side. Of course as spanish, catalan has its dialects, there is even a city in Italy where catalan is spoken, and I think there is another in France with its varieties. But a catalan native speaker could tell you more and more accurately.

It's quite easy to spot Catalan because they use different letters from Castellano, for example:

1) no "ñ", Cataluña versus Catalunya

2) ç, feliz versus feliç

3) x, caja versus caixa

4) l·l (yes, that's actually one letter)

etc