It depends on where do you want to travel, and if you want to be able to speak to everybody in Spain and some countries in South America. As they said, Catalan is a separate language from Spanish. It sounds more or less similar to it, but people not accustomed to hear it can take longer to start understanding it. It's spoken mainly in Cataluña, but also in many other places, both inside and outside Spain. Inside Spain you can hear Catalan in Valencia (really, they speak a variant of Catalan there), some places in Murcia, Baleares and Aragón, and outside Spain it's spoken in Andorra, some regions in Italy and France. Catalan is really more similar to French than to Spanish in its vocabulary, but its pronunciation is nearer to the Spanish one. On the other side, Spanish is understood in all regions in Spain, in most South American countries, and many many other places, being the second most natively spoken language in the world, so certainly it's worth learning Spanish. Being spoken in so many regions, you can be sure there are many variants of Spanish out there. Spain's Spanish can be a good reference, but South America countries can use a very different vocabulary in many informal situations. I hope that helps.

The difference is that Spanish is spoken in all the country whereas Catalian is only Spoken in Cataluña! So if you want to travel to Spain you should learn Spanish!

Catalan is a sort of dialect/could be an independant language from Spanish, spoken in Cataluña, an area in eastern Spain; as Paula said, you don't really need to speak it when going to Spain because even in Cataluña they speak Castellano: the official Spanish spoken in Spain. In Latin America, they speak Yéismo, a form of Castellano which is not fundamentally different - just a phonetical difference, with the occasional different nuance as well. What you refer to as Spanish is Castellano, so learn that.

The Catalian is like a lenguaje very similar to Spanish. If you want to travel to Spain, learn to speak Catalian is not necesary, because here everybody can speak Spanish although they live in Cataluña.

What do you mean "Catalian"? Is it "Castilian" or "Catalan"?