The company might pay for some or all of the following: movers to move your belongings and autos, your travel expenses, storage of your belongings while you buy a house, temporary living while you buy a house, meals during travel, moving your pets, closing costs and/or realtor commissions when selling your present home, closing costs and/or inspections when buying a house... With his current job, my DH got all of that and the company paid the points on our new mortgage to buy down the interest rate and provided "gross up" (you have to pay fed & state taxes on much of this stuff -- "gross up" is when the company pays for the stuff AND gives you enough money to pay the taxes too).
The downside with relocation assistance is that you'll have to sign a document agreeing to repay the relocation money if you leave the company within a certain amount of time. With some companies, it's one year and with others it's two years.
Edited to add: Make sure you get the details of the "relocation assistance" in any offer package. A good relocation package could save you thousands of dollars. (On our last move, I think the relocation package ended up being on the order of $25-$30k.)