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
